Fix user.onDisconnect not being run on disconnectAll()

- Guests banned on join will now be immediately destroyed.
- Unauthenticated authorities will now lose their group after banned.
Not necessary... <code>this.connections</code> is empty there due to <code>User#onDisconnect</code>, which is called from <code>Connection#onDisconnect</code> <- <code>Connection#destroy</code>.

If you still want to keep the safeguard, however, we should probably be saving logs when the user has any connections here at this point.