[JRO] Move filter(s). Rename UserDecorator

* Delete unused features. Move filters to messageboard preferences
* Allow user to select their favorite "filter" per messageboard
* Rename UserDecorator to UserExtender because that's what it's doing.
* Move the recently_active functionality from messageboard to user as a class method. We're looking for users so it should be related to the concept of user, not message boards.