User options were serialized at the root level of CurrentUserSerializer,
but UserSerializer has a user_option field. This inconsistency caused
issues in the past because user_option fields had to be duplicated on
the frontend.
This removes all custom controllers and redis/messagebus logic from discourse-presence, and replaces it with core's new PresenceChannel system.
All functionality should be retained. This implementation should scale much better to large numbers of users, reduce the number of HTTP requests made by clients, and reduce the volume of messages on the MessageBus.
For more information on PresenceChannel, see 31db8352
