RCFilters: Make 'days' and 'limit' sticky
- Each has a hidden preference to override the preferences value
- Each value is different between Watchlist and RecentChanges
- If RCFilters is enabled, the preferences for Watchlist are hidden from the preference page.
- If RCFilters is enabled, the 'rcdays' preference, in RecentChanges, is hidden.
- 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.