Enable RCFilters app on Watchlist

Enable RCFilters app on Watchlist

* Add classes prefixed with cl (for ChangesList)
  to both RC and WL so that the JS app can locate
  similar elements using the same selectors

* Make saved queries preference name configurable
  so that RC and WL use different preferences

* Move some code from SpecialRecentchanges.php to
  its base class so it's accessible to SpecialWatchlist.php

To use the RCFilters app on WL, append ?rcfilters=1 to the URL

Bug: T171132
Bug: T171218
Change-Id: If7c63284224df24fa02022b4ea74bef116f28ca6