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.

This supersedes/fixes-up this Pull Request:
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/719

The Jitsi Web and JVB containers now (in build 5142) always
start by bulding their own default configuration
(`config.js` and `sip-communicator.properties`, respectively).

The fact that we were generating these files ourselves was no longer of use,
because our configuration was thrown away in favor of the one created
by the containers on startup.

With this commit, we're completely redoing things. We no longer
generate these configuration files. We try to pass the proper
environment variables, so that Jitsi services can generate the
configuration files themselves.

Besides that, we try to use the "custom configuration" mechanism
provided by Jitsi Web and Jitsi JVB (`custom-config.js` and
`custom-sip-communicator.properties`, respectively), so that
we and our users can inject additional configuration.

Some configuration options we had are gone now. Others are no longer
controllable via variables and need to be injected using
the `_config_extension` variables that we provide.

The validation logic that is part of the role should take care
to inform people about how to upgrade (if they're using some custom
configuration, which needs special care now). Most users should not
have to do anything special though.