don't bind to any host port if nginx_proxy is used
only bind to localhost if it's not used

.. and other documentation improvements.

Also includes the dashboards for Synapse and for Node Exporter.

Again has only been tested on debian amd64 so far, but the grafana docker image is available for arm64 and arm32. Nice.

Basic system stats, to show stuff the synapse metrics
can't show such as resource usage by bridges, etc

Seems to work fine as well.

This too has only been tested on debian amd64 so far

Initial attempt. Seems to work fine.

Only tested on debian amd64 so far

I felt that adding another variable was probably going to be the easiest way to do this. I may end up adding another variable to enable this feature, for consistency with some of the other things.

In short, this makes Synapse a 2nd class citizen,
preparing for a future where it's just one-of-many homeserver software
options.

We also no longer have a default Postgres superuser password,
which improves security.

The changelog explains more as to why this was done
and how to proceed from here.

I had intentionally held it back in 39ea3496a4
until:
- it received more testing (there were a few bugs during the
migration, but now it seems OK)
- this migration guide was written

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/793

That bridge only supports its own file-based database,
so preparing a Postgres database for it is pointless.

The answer to these is: it's good to have them in both places.
The role defines the obvious things it depends on (not knowing
what setup it will find itself into), and then
`group_vars/matrix_servers` "extends" it based on everything else it
knows (the homeserver being Synapse, whether or not the internal
Postgres server is being used, etc.)

Otherwise the postgres upgrade fails with the following error:

Unexpected templating type error occurred on ({{
  [matrix_postgres_connection_username]
  +
  matrix_postgres_additional_databases|map(attribute='username')
}}
): can only concatenate list (not "generator") to list

Some Ansible installations choke on it, it seems.
Similar to 9f00970c90

We've hit this problem before as well. Certain Ansible installations
choke on it.

We were only defining this in `group_vars/matrix_servers`, which is
inconsistent with how we normally do things.

Now that 0.7.2 is out, the Docker image supports Postgres
and we can do the (SQLite -> Postgres) migration.

I've also found out that we needed to fix up the `tokens.ex_date` column
data type a bit to prevent matrix-registration from raising exceptions
when comparing `datetime.now()` with `ex_date` coming from the database.

Example:

> File "/usr/local/lib/python3.8/site-packages/matrix_registration/tokens.py", line 58, in valid
> expired = self.ex_date < datetime.now()
> TypeError: can't compare offset-naive and offset-aware datetimes

We were running into conflicts, because having initialized
the roles (users) and databases, trying to import leads to
errors (role XXX already exists, etc.).

We were previously ignoring the Synapse database (`homeserver`)
when upgrading/importing, because that one gets created by default
whenever the container starts.

For our additional databases, it's a similar situation now.
It's not created by default as soon as Postgres starts with an empty
database, but rather we create it as part of running the playbook.

So we either need to skip those role/database creation statements
while upgrading/importing, or to avoid creating the additional database
and rely on the import for that. I've gone for the former, because
it's already similar to what we were doing and it's simpler
(it lets `setup_postgres.yml` be the same in all scenarios).

Auto-migration and everything seems to work. It's just that
matrix-registration cannot load the Python modules required
for talking to a Postgres database.

Tracked here: https://github.com/ZerataX/matrix-registration/issues/44

Until this gets fixed, we'll continue default to 'sqlite'.

This has been tested and appears to work.

Looks like there are some that I missed in 087dbe4ddc

I was thinking that it makes sense to be more specific,
and using `_postgres_` also separated these variables
from the `_database_` variables that ended up in bridge configuration.

However, @jdreichmann makes a good point
(https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/740#discussion_r542281102)
that we don't need to be so specific and can allow for other engines (like MySQL) to use these variables.

The only one that remains is `matrix_synapse_database_password`, but
that's something old and should be dealt with separately in the future
(unless it remains as it is).

Related to 9bf8ce878e.

Should have been done during 2a99e84b5bfd9f39, but I've missed it.

Because of this, 9bf8ce878e was also incomplete and would have caused
an error.

`matrix_SERVICE_database_engine` remains as it is - the only one with a
`_database_` infix. Postgres-related ones move to `_postgres_`.

Using the result of `password_hash` works for creating them,
but authentication seems to be failing with some tools like pgloader.

It's possible that we're not escaping things properly somewhere.
Ideally, it'd be nice to solve that. But the easier (and still
relatively safe/good) solution is to just turn that password hash
into a UUID that's safe for passing around without worrying about
escaping.