Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1057

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/1084

Conversation: https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1083#discussion_r638671860

Signed-off-by: Ahmad Haghighi <haghighi@fedoraproject.org>

Signed-off-by: Ahmad Haghighi <haghighi@fedoraproject.org>

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1024

I changed the conditional statement in prosody systemd template to bind the localhost port by default if people have set ```matrix_nginx_proxy_enabled == false ```.
Hopefully that should make it the default behaviour now.

Added:
 - Conditional localhost Port bindings for Jitsi Prosody systemd template
- Added variable to main.yml to allow overriding from vars.yml

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1061
and https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1057

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1070

Related discussion here: 1ab507349c (commitcomment-51108407)

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1070

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1062

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1062

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1057

Not sure if this is beneficial though.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1057

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1058

We may try to capture such calls and return a friendlier response (HTML
or JSON) saying "The Synapse Admin API is not enabled", but that may not
be desirable.

For now, we stick to what "upstream" recommends: "simply
don't proxy these APIs", which should lead to the same kind of 404 that
we have now.
See here: 6660912226/docs/reverse_proxy.md (synapse-administration-endpoints)

Related to https://github.com/devture/exim-relay/pull/9

Related to https://github.com/Awesome-Technologies/synapse-admin/issues/132

We should switch back when >0.8.0 gets released.