Using a separate directory allows easier backups
(only need to back up the Ansible playbook configuration and the
bridge's `./data` directory).

The playbook takes care of migrating an existing database file
from the base directory into the `./data` directory.

In the future, we can also mount the configuration read-only,
to ensure the bridge won't touch it.
For now, mautrix-facebook is keen on rebuilding the `config.yaml`
file on startup though, so this will have to wait.