.. and prevent variable name overlap with `matrix-postgres` for the
other variables as well.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/868

See these last commits:

tulir/mautrix-signal@4fc34330c1f6947aece67863b0d04da34c776f80

tulir/mautrix-signal@64bc5c36a509ba435a0b01cf44afb1b5d2642efd

tulir/mautrix-signal@ddda1666d41d28750cc59d070e4388b24add6ad9

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/963

This also simplifies Prerequisites, which is great.

It'd be nice if we were doing these checks in some optional manner
and reporting them as helpful messages (using
`matrix_playbook_runtime_results`), but that's more complicated.
I'd rather drop these checks completely.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/970

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/970

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/969

This adds https://github.com/matrix-org/matrix-doc/pull/2409 to the
appservice registrations, enabling synapse to push EDUs to appservices.

That is:
- enabled in the role by default
- disabled in the compilation (playbook), if considered an optional
component

This variable was previously undefined in the role and was only getting
defined via `group_vars/matrix_servers`.

We now properly initialize it (and its good default value) in the role
itself.

Fixes a regression introduced in ffe649a240

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/958

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/958

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/683

Fixes a regression caused by a5ee39266c.

If the user id and group id were different than 991:991
(which used to be a hardcoded default for us long ago),
there was a mismatch between what Synapse was trying to use (991:991)
and what it was actually started with (in `--user=..`). It was then
trying to change ownership, which was failing.

This was mostly affecting newer installations which were not using the
991:991 defaults we had long ago (since a1c5a197a9).

This allows us to benefit from helpful things it does for us,
like enabling jemalloc: https://github.com/matrix-org/synapse/pull/8553

We weren't going through `start.py` before, because it was causing some
conflict with our `docker run --user=...` stuff, but it doesn't seem
to be a problem anymore.

Having done this, we won't need to do things like
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/941
anymore.

- removed build_database_list.yml

- add matrix_postgres_backup_databases to be build on top of matrix_postgres_additional_databases
- POSTGRES_DB is now directly set from matrix_postgres_backup_databases while building the templates/env-postgres-backup.j2

- adjusted env-postgres-backup.j2 to use the new role specific variables