Fix apt message: docker doesn't support arch 'i386'

Related to #223 (Github Pull Request)

Update synapse (1.1.0 -> 1.2.0)

Update nginx (1.17.1 -> 1.17.2)

Upgrade riot-web (1.2.4 - 1.3.0)

introduce configuration to change riot branding / title

Looks like these client ids are actually integers,
but unless we pass them as a string, the bridge would complain with
an error like:

    {"field":"data.auth.clientID","message":"is the wrong type","value":123456789012345678,"type":"string","schemaPath":["properties","auth","properties","clientID"]}

Explicitly-casting to a string should fix the problem.

The Discord bridge should probably be improved to handle both ints and
strings though.

Fixes #216 (Github Issue).

Update to riot 1.2.3

We do restart Synapse explicitly, but some other services
(bridges, matrix-corporal, ..) may not restart sometimes.

It's best to restart all services explicitly.

Regression since 174a6fcd1b3, #204 (Github Pull Request),
which only affects new servers.

Old servers which had their passkey.pem file relocated were okay.

Bump nginx version to 1.17.1

ef5e4ad061 intentionally makes us conform to
the logging format suggested by the official Docker image.

Reverting this part, because it's uglier.

This likely should be fixed upstream as well though.

Somewhat related to #213 (Github Pull Request).

We've been moving in the opposite direction for quite a long time.
All services should just leave logging to systemd's journald.

Fixes a regression introduced during the upgrade to
Synapse v1.1.0 (in 2b3865ceea).

Since Synapse v1.1.0 upgraded to Python 3.7
(https://github.com/matrix-org/synapse/pull/5546),
we need to use a different modules directory when mounting
password provider modules.

Fixes a problem introduced by da6edc9cba.

Related to #145 (Github Pull Request).

This is a new feature of Synapse v1.1.0.

Discussed in #145 (Github Pull Request).

Make Discord bridge configuration playbook-managed

Well, `config.yaml` has been playbook-managed for a long time.
It's now extended to match the default sample config of the Discord
bridge.

With this patch, we also make `registration.yaml` playbook-managed,
which leads us to consistency with all other bridges.

Along with that, we introduce `./config` and `./data` separation,
like we do for the other bridges.

According to
https://passlib.readthedocs.io/en/stable/lib/passlib.hash.sha512_crypt.html:

> salt (str) – Optional salt string. If not specified, one will be autogenerated (this is recommended).
> If specified, it must be 0-16 characters, drawn from the regexp range [./0-9A-Za-z].

Until now, we were using invalid characters (like `-`). We were also
going over the requested length limit of 16 characters.

This is most likely what was causing `ValueError` exceptions for some people,
as reported in #209 (Github Issue).
Ansible's source code (`lib/ansible/utils/encrypt.py`) shows that Ansible tries
to use passlib if available and falls back to Python's `crypt` module if not.
For Mac, `crypt.crypt` doesn't seem to work, so Ansible always requires passlib.

Looks like crypt is forgiving when length or character requirements are
not obeyed. It would auto-trim a salt string to make it work, which means
that we could end up with the same hash if we call it with salts which aer only
different after their 16th character.

For these reasons (crypt autotriming and passlib downright complaining),
we're now using shorter and more diverse salts.

Update nginx and postgres images to latest versions