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.

We do this for 2 reasons:

- so we can control things which are not controllable using environment
variables (for example `stunServers` in jitsi/web, since we don't wish
to use the hardcoded Google STUN servers if our own Coturn is enabled)

- so playbook variable changes will properly rebuild the configuration.
When using Jitsi environment variables, the configuration is only built
once (the first time) and never rebuilt again. This is not the
consistent with the rest of the playbook and with how Ansible operates.
We're not perfect at it (yet), because we still let the Jitsi containers
generate some files on their own, but we are closer and it should be
good enough for most things.

Related to #415 (Github Pull Request).