Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

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.

> Invalid data passed to 'loop', it requires a list, got this instead: matrix_postgres_additional_databases. Hint: If you passed a list/dict of just one element, try adding wantlist=True to your lookup invocation or use q/query instead of lookup.

Well, or working around it, as I've done in this commit (which seems
more sane than `wantlist=True` stuff).

To avoid needing to have `jq` installed on the machine, we could:
- try to run jq in a Docker container using some small image providing
that
- better yet, avoid `jq` altogether

We'll most likely use one that matches the database name, but
it's better to have it configurable.

Moving it above the "uninstalling" set of tasks is better.
Extracting it out to another file at the same time, for readability,
especially given that it will probably have to become more complex in
the future (potentially installing `jq`, etc.)