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.

I don't use this bridge, so this is completely untested.

Regression since 2d99ade72f and 9bf8ce878e94, respectively.

When SQLite is to be used, these bridges expect an `sqlite://`
connection string, and not a plain file name (path), like Appservice
Discord and mautrix-whatsapp do.

I don't use this bridge, so this is completely untested.

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.

I don't use this bridge, so this is completely untested.

Instead of passing the connection string, we can now pass a name of a
variable, which contains a connection string.

Both are supported for having extra flexibility.

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

This can be used by various bridges, etc., to import an SQLite
(or some other supported) database into Postgres.

Since we'll likely have generic SQLite database importing
via [pgloader](https://pgloader.io/) for migrating bridge
databases from SQLite to Postgres, we'd rather avoid
calling the "import Synapse SQLite database" command
as just `--tags=import-sqlite-db`.

Similarly, for the media store, we'd like to mention that it's
related to Synapse as well.

We'd like to be more explicit, so as to be less confusing,
especially in light of other homeserver implementations
coming in the future.

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.

People can toggle between them now. The playbook also defaults
to using SQLite if an external Postgres server is used.

Ideally, we'd be able to create databases/users in external Postgres
servers as well, but our initialization logic (and `docker run` command,
etc.) hardcode too many things right now.

Being more explicit sounds better.

While these modules are really nice and helpful, we can't use them
for at least 2 reasons:

- for us, Postgres runs in a container on a private Docker network
(`--network=matrix`) without usually being exposed to the host.
These modules execute on the host so they won't be able to reach it.

- these modules require `psycopg2`, so we need to install it before
using it. This might or might not be its own can of worms.