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

I've been thinking of doing before, but haven't.

Now that the Whatsapp bridge does it (since 4797469383),
it makes sense to do it for all other bridges as well.
(Except for the IRC bridge - that one manages most of registration.yaml by itself)

Doesn't matter much, but it makes it consistent with the other bridges.

Whatsapp config playbook managed

Disable appservice-irc log files

- changelog entry for commit 4797469383

- following spantaleev transition of the telegram brigde
- adding a validate_config task

appservice-irc doesn't have permission to create files in its project
directory and the intention is to log to the console, anyway. By
commenting out the file names, appservice-irc won't attempt to open the
files.

Make IRC bridge configuration entirely managed by the playbook

This means we need to explicitly specify a `media_url` now,
because without it, `url` would be used for building public URLs to
files/images. That doesn't work when `url` is not a public URL.

Regression since 4e8543ce21

Until now, if `--tags=setup-synapse` was used, bridge tasks would not
run and bridges would fail to register with the `matrix-synapse` role.
This means that Synapse's configuration would be generated with an empty
list of appservices (`app_service_config_files: []`).

.. and then bridges would fail, because Synapse would not be aware of
there being any bridges.

From now on, bridges always run their init tasks and always register
with Synapse.

For the Telegram bridge, the same applies to registering with
matrix-nginx-proxy. Previously, running `--tags=setup-nginx-proxy` would
get rid of the Telegram endpoint configuration for the same reason.
Not anymore.

Note: https://www.us-cert.gov/ncas/current-activity/2019/06/13/Exim-Releases-Security-Patches

That said, I don't believe we've been affected.
Not in a bad way at least, because:
- we run exim as non-root and capabilities dropped
- we run exim in a private Docker network with known trusted relayers
(Synapse and mxisd)

I've not found this welcome bot to work at all in my previous attempts.
It would simply not reply, even though federation works.

It seems like this is also a potential privacy issue, as per
https://gist.github.com/maxidorius/5736fd09c9194b7a6dc03b6b8d7220d0

With most people on Synapse v0.99+ and Synapse v1.0 now available,
we should no longer try to be backward compatible with Synapse 0.34,
because this just complicates the instructions for no good reason.

Added a basic guide on migrating to another server

Changes to the original are:
- it tells people to stop and disable services, so that:
   - services won't be running while you are copying files
   - services won't accidentally start again later
- it does the file-copying in 1 step
- it does copying before running `--tags=setup-all`, so that existing files (SSL certificates, etc.) can be reused. Otherwise, the playbook starts from a blank slate, retrieves them anew, generates new signing keys anew, etc. Only to have those replaced by your own old backup later.
- it mentions DNS changes
- combines `--tags=setup-all,start` into a single step, thanks to the files being already copied

Allow default room version to be configured

Using `|to_json` on a string is expected to correctly wrap it in quotes (e.g. `"4"`).
Wrapping it explicitly in double-quotes results in undesirable double-quoting (`""4""`).

Upgrade mxisd (1.4.4 -> 1.4.5)

We do use some `:latest` images by default for the following services:
- matrix-dimension
- Goofys (in the matrix-synapse role)
- matrix-bridge-appservice-irc
- matrix-bridge-appservice-discord
- matrix-bridge-mautrix-facebook
- matrix-bridge-mautrix-whatsapp

It's terribly unfortunate that those software projects don't release
anything other than `:latest`, but that's how it is for now.

Updating that software requires that users manually do `docker pull`
on the server. The playbook didn't force-repull images that it already
had.

With this patch, it starts doing so. Any image tagged `:latest` will be
force re-pulled by the playbook every time it's executed.

It should be noted that even though we ask the `docker_image` module to
force-pull, it only reports "changed" when it actually pulls something
new. This is nice, because it lets people know exactly when something
gets updated, as opposed to giving the indication that it's always
updating the images (even though it isn't).

Add config option to be able to access database outside of container