added docs, added seperate service for database, pr comments

Related to #454 (Github Pull Request).

Related to #425 (Github Pull Request).

This fixes an incorrect indentation in the database specification for
appservice-irc which caused matrix-appservice-irc to refuse to start
with the remarkably unhelpful error message:

```
ERROR:CLI Failed to run bridge.
```

This also updates doc links to the new matrixdotorg repo because the
tedomum repo contains out-of-date documentation.

Related to #339 (Github Pull Request).

Bridge is setup to work on the matrix side with this, but the discord invite link is not automatically generated.

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(https://hub.docker.com/r/nginxinc/nginx-unprivileged is outdated).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.

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`).

Adds support for managing certificates manually and for
having the playbook generate self-signed certificates for you.

With this, Let's Encrypt usage is no longer required.

Fixes Github issue #50.

The new image is built in a much better way (2-stage build)
and is 10x smaller.

In terms of Goofys version recency, it's about the same..
Both images (and others alike) seem to not use version tags,
but rather some `:latest` (master), with ewoutp/goofys being a bit
more recent than clodproto/goofys.

Not using version tags is good (in this case),
because the last Goofys release seems to be from about a year ago
and there had been a bunch of bugfixes afterwards.

This reverts commit 8d774db3bc.

Docker is released in the Docker CE stable repository now.

Additionally, it's version 18.03, which doesn't suffer
any of the problems we've observed with 18.05 (edge/nightly).