This commit introduces a new role that downloads and installs the
prometheus community postgres exporter  https://github.com/prometheus-community/postgres_exporter.
A new credential is added to matrix_postgres_additional_databases that
allows the exporter access to the database to gather statistics.
A new dashboard was added to the grafana role, with some refactoring
to enable the dashboard only if the new role is enabled.
I've included some basic instructions for how to enable the role in
the Docs section.

In terms of testing, I've tested enabling the role, and disabling
it to make sure it cleans up the container and systemd role.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1073

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1047

This reverts commit 1fb54a37cb.

Seems like it's been pulled or something. It used to exist, but not
anymore. Not sure what's going on.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1017

Related to
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1010

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1010

This reverts commit f825c7c263.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1010

Expected to have regressed after https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1008

This patch comes with its own downsides (as described in the comments
for matrix_prometheus_node_exporter_container_http_host_bind_port),
but at least there's:
- no security issue
- metrics remain readable from matrix-prometheus (even if the network metrics are inaccurate)

A better patch is certainly welcome.

Not sure why this had been done in the first place.
It doesn't make any sense.

There's no relation between matrix-nginx-proxy and
prometheus-node-exporter.

This was causing a port conflict when `matrix_nginx_proxy_enabled: false`.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/983

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

Some people have reported Dimension failing to contact
matrix-nginx-proxy/Synapse on startup. This attempts to
delay Dimension startup some more.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/966

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

We don't point them straight to Synapse anymore, but rather
to matrix-nginx-proxy, which forwards to Synapse (or something else).

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

- 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

Mostly affects people who disable the integrated `matrix-nginx-proxy`.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/456
and more specifically 4d62a75f6f.

People who were disabling matrix-nginx-proxy (in favor of their own
nginx webserver) and also overriding `matrix_federation_public_port`,
found that the generated nginx configuration still hardcoded `8448`,
which forced their nginx server to use that, regardless of the fact
that `matrix_federation_public_port` was pointing elsewhere.

We now allow for the in-container federation port to be configurable,
and also automatically wire things properly.

Related to: https://matrix.org/blog/2021/02/18/synapse-1-27-0-released#dropping-armv7-docker-images

The official Etherpad Docker image has no support for sqlite3 databases.

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.