Re-fetch config + register notifications after setting changes #16

When the user changes the server URL, everything should be re-fetched (make sure that firebase initialization works if we call it again or maybe we'll have to cleanup first - we should probably also delete the old token to make sure we won't receive notifications anymore if possible).

If the list of linked browsers changes, the register-notification-endpoint should be called (so that either the new linked browser can send a notification or so that unlinked browsers can no longer send notifications)