Add support for matrix-appservice-irc

Regression since 299a8c4c7c

Make containers start as non-root, without capabilities and with a read-only filesystem

Seems like we unintentionally removed the mounting of certificates
(the `/matrix-config` mount) as part of splitting the playbook into
roles in 51312b8250.

It appears that those certificates weren't necessary for coturn to
funciton though, so we might just get rid of the configuration as well.

Continuation of 316d653d3e

We run containers as a non-root user (no effective capabilities).

Still, if a setuid binary is available in a container image, it could
potentially be used to give the user the default capabilities that the
container was started with. For Docker, the default set currently is:
- "CAP_CHOWN"
- "CAP_DAC_OVERRIDE"
- "CAP_FSETID"
- "CAP_FOWNER"
- "CAP_MKNOD"
- "CAP_NET_RAW"
- "CAP_SETGID"
- "CAP_SETUID"
- "CAP_SETFCAP"
- "CAP_SETPCAP"
- "CAP_NET_BIND_SERVICE"
- "CAP_SYS_CHROOT"
- "CAP_KILL"
- "CAP_AUDIT_WRITE"

We'd rather prevent such a potential escalation by dropping ALL
capabilities.

The problem is nicely explained here: https://github.com/projectatomic/atomic-site/issues/203

This is a known/intentional regression since f92c4d5a27.

The new stance on this is that most people would not have
dnspython, but may have the `dig` tool. There's no good
reason for not increasing our chances of success by trying both
methods (Ansible dig lookup and using the `dig` CLI tool).

Fixes #85 (Github issue).

This makes all containers (except mautrix-telegram and
mautrix-whatsapp), start as a non-root user.

We do this, because we don't trust some of the images.
In any case, we'd rather not trust ALL images and avoid giving
`root` access at all. We can't be sure they would drop privileges
or what they might do before they do it.

Because Postfix doesn't support running as non-root,
it had to be replaced by an Exim mail server.

The matrix-nginx-proxy nginx container image is patched up
(by replacing its main configuration) so that it can work as non-root.
It seems like there's no other good image that we can use and that is up-to-date
(https://hub.docker.com/r/nginxinc/nginx-unprivileged is outdated).

Likewise for riot-web (https://hub.docker.com/r/bubuntux/riot-web/),
we patch it up ourselves when starting (replacing the main nginx
configuration).
Ideally, it would be fixed upstream so we can simplify.

We do match the defaults anyway (by default that is),
but people can customize `matrix_user_uid` and `matrix_user_uid`
and it wouldn't be correct then.

In any case, it's better to be explicit about such an important thing.

Fixes #83 (Github issue)

A long-running import task may hit the SSH timeout value
and die. Using async is supposed to improve reliability
in such scenarios.

If this is a brand new server and Postgres had never started,
detecting it before we even start it is not possible.

This moves the logic, so that it happens later on, when Postgres
would have had the chance to start and possibly initialize
a new empty database.

Fixes #82 (Github issue)

It hasn't mattered much to have these so far, but
it's probably a good idea to have them.

It worked without it too, but doing this is more consistent with the
mxisd recommendations.

Implements #77 (Github issue).

Make roles more independent of one another

The matrix-nginx-proxy role can now be used independently.
This makes it consistent with all other roles, with
the `matrix-base` role remaining as their only dependency.

Separating matrix-nginx-proxy was relatively straightforward, with
the exception of the Mautrix Telegram reverse-proxying configuration.
Mautrix Telegram, being an extension/bridge, does not feel important enough
to justify its own special handling in matrix-nginx-proxy.

Thus, we've introduced the concept of "additional configuration blocks"
(`matrix_nginx_proxy_proxy_matrix_additional_server_configuration_blocks`),
where any module can register its own custom nginx server blocks.

For such dynamic registration to work, the order of role execution
becomes important. To make it possible for each module participating
in dynamic registration to verify that the order of execution is
correct, we've also introduced a `matrix_nginx_proxy_role_executed`
variable.

It should be noted that this doesn't make the matrix-synapse role
dependent on matrix-nginx-proxy. It's optional runtime detection
and registration, and it only happens in the matrix-synapse role
when `matrix_mautrix_telegram_enabled: true`.

With this change, the following roles are now only dependent
on the minimal `matrix-base` role:
- `matrix-corporal`
- `matrix-coturn`
- `matrix-mailer`
- `matrix-mxisd`
- `matrix-postgres`
- `matrix-riot-web`
- `matrix-synapse`

The `matrix-nginx-proxy` role still does too much and remains
dependent on the others.

Wiring up the various (now-independent) roles happens
via a glue variables file (`group_vars/matrix-servers`).
It's triggered for all hosts in the `matrix-servers` group.

According to Ansible's rules of priority, we have the following
chain of inclusion/overriding now:
- role defaults (mostly empty or good for independent usage)
- playbook glue variables (`group_vars/matrix-servers`)
- inventory host variables (`inventory/host_vars/matrix.<your-domain>`)

All roles default to enabling their main component
(e.g. `matrix_mxisd_enabled: true`, `matrix_riot_web_enabled: true`).
Reasoning: if a role is included in a playbook (especially separately,
in another playbook), it should "work" by default.

Our playbook disables some of those if they are not generally useful
(e.g. `matrix_corporal_enabled: false`).

We've previously changed a bunch of lists in `homeserver.yaml.j2`
to be serialized using `|to_nice_yaml`, as that generates a more
readable list in YAML.

`matrix_synapse_federation_domain_whitelist`, however, couldn't have
been changed to that, as it can potentially be an empty list.

We may be able to differentiate between empty and non-empty now
and serialize it accordingly (favoring `|to_nice_yaml` if non-empty),
but it's not important enough to be justified. Thus, always
serializing with `|to_json`.

Fixes #78 (Github issue)

Fix scalar widgets

Riot-web parses integrations_widgets_urls as a list, thus causing it to incorrectly think Scalar widgets are non-Scalar and not passing the scalar token

Split playbook into multiple roles

Make the mode of the base path configurable

As suggested in #63 (Github issue), splitting the
playbook's logic into multiple roles will be beneficial for
maintainability.

This patch realizes this split. Still, some components
affect others, so the roles are not really independent of one
another. For example:
- disabling mxisd (`matrix_mxisd_enabled: false`), causes Synapse
and riot-web to reconfigure themselves with other (public)
Identity servers.

- enabling matrix-corporal (`matrix_corporal_enabled: true`) affects
how reverse-proxying (by `matrix-nginx-proxy`) is done, in order to
put matrix-corporal's gateway server in front of Synapse

We may be able to move away from such dependencies in the future,
at the expense of a more complicated manual configuration, but
it's probably not worth sacrificing the convenience we have now.

As part of this work, the way we do "start components" has been
redone now to use a loop, as suggested in #65 (Github issue).
This should make restarting faster and more reliable.

Update README to link to bubuntux/riot-web

The new container image is about 20x smaller in size, faster to start up, etc.

This also fixes #26 (Github issue).

Regression since 9a9b7383e9.