RCFilters: Make 'days' and 'limit' sticky

RCFilters: Make 'days' and 'limit' sticky

* Each has a hidden preference to override the preferences value
* Each value is different between Watchlist and RecentChanges
* rcfilters-limit is updated when rclimit is changed
* Not conditionally hiding the rcdays, watchlistdays and wllimit yet
  because hide-if's behavior is annoying

Bonus:
* Add a static method to check whether RCFilters UI is enabled
  and enabled by default. Adjust the call for Watchlist which
  checks a slightly different configuration setup.

Bug: T174415
Change-Id: Ib933de3a3f9e876924386e80f315506f60f8af54