Continuation of 1f0cc92b33.

As an explanation for the problem:
when saying `localhost` on the host, it sometimes gets resolved to `::1`
and sometimes to `127.0.0.1`. On the unfortunate occassions that
it gets resolved to `::1`, the container won't be able to serve the
request, because Docker containers don't have IPv6 enabled by default.

To avoid this problem, we simply prevent any lookups from happening
and explicitly use `127.0.0.1`.

Turn off IPv6 when using your own Nginx server

Docker apparently doesn't like IPv6.

Enable exposure of Prometheus metrics.

Appservice IRC database files must be stored on permanent storage

Regression since 51312b82

This supersedes #59 (Github Pull Request),
which was greatly beneficial in creating our sample Apache configuration.

This was mentioned in #27 (Github Pull Request),
but it's just now that the nginx Docker image actually supports
TLS v1.3 and we can enable it.

Related to #88 (Github Pull Request)

Adding .editorconfig.

This reverts commit 0dac5ea508.

Relying on pyOpenSSL is the Ansible way of doing things, but is
impractical and annoying for users.

`openssl` is easily available on most servers, even by default.
We'd better use that.

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).