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.

As of docker-jitsi-meet stable-6433 [1], `/config/interface_config.js`
is regenerated on every boot. The correct way to modify the interface
config is now via `/config/custom-interface_config.js`, which is
appended to a default copy of `interface_config.js` by
`/etc/cont-init.d/10-config` on every boot of the docker image.

Given that `interface_config.js` is considered deprecated by upstream
(all options will eventually be moved to `config.js`), we also deprecate
the `matrix_jitsi_web_interface_config_*` variables in favour of
`matrix_jitsi_web_custom_interface_config_extension`.

[1] https://github.com/jitsi/docker-jitsi-meet/blob/stable-6433/CHANGELOG.md#stable-6433