Fix #2720.

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

Comments taken from https://github.com/matrix-org/matrix-appservice-slack/blob/develop/config/config.sample.yaml

We should probably reconcile our configuration with that one and include comments for other fields as well.

It is very confusing to debug why messages only go from Matrix to Slack
but not from Slack to Matrix. RTM should be enabled by default, as
that's the recommended way to make this work.

If a service is enabled, a database for it is created in postgres with a uniqque password. The service can then use this database for data storage instead of relying on sqlite.