Prevent users from external servers to join public channels

[nesium]

In the current implementation I've configured Public Rooms as open MUC rooms, meaning everyone can join. We've talked about this briefly that this would allow users from foreign servers to join as well, which is not desireable. Another option could be to configure these as member-only rooms similar to Groups and let the user try to register themselves. The server MUC module could then i.e. automatically grant the request for users from the same domain and forward the request to an admin for users from a foreign domain.