People can toggle between them now. The playbook also defaults
to using SQLite if an external Postgres server is used.
Ideally, we'd be able to create databases/users in external Postgres
servers as well, but our initialization logic (and `docker run` command,
etc.) hardcode too many things right now.
Well, `config.yaml` has been playbook-managed for a long time.
It's now extended to match the default sample config of the Discord
bridge.
With this patch, we also make `registration.yaml` playbook-managed,
which leads us to consistency with all other bridges.
Along with that, we introduce `./config` and `./data` separation,
like we do for the other bridges.
