update to v0.2.0

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1140

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1131

The new one is based on Alpine 3.14, instead of Alpine 3.13.

https://github.com/vector-im/element-web#configuration-best-practices

Should have been part of 9daeb39710

https://github.com/vector-im/hydrogen-web/releases/tag/v0.1.57

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1108

Without this the hydrogen task would fail for Ansible 2.7 even when hydrogen is not enabled.

Update hydrogen-web version to v0.1.56

Fixes running the container as an unprivileged user.

It's mildly annoying when trying to execute these scripts while logged
in as a regular user, as the missing execute permissions will hinder
autocompletion even when trying to use with sudo.

These shell scripts don't contain secrets, but may fail when ran by a
regular user. The failure is due to the lack of access to the /matrix
directory, and does not result in any damage.

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.