Make SpecialWatchlist extend SpecialRecentChanges (temporarily)

Make SpecialWatchlist extend SpecialRecentChanges (temporarily)

Later both will be made to inherit from a base class, but this
makes some things simpler for now.

* `class SpecialWatchlist extends SpecialRecentChanges { … }`
* Fixes to both classes to ensure nothing breaks
* Remove redundant SpecialWatchlist#getGroupName()

No functional changes yet; just the framework change. Refactoring
follows in subsequent commits, with a base class for both of
these being introduced in I5855074deed52fd7492f033c8a8fced8af647602.

Bug: 48641
Change-Id: I394ab71bae876684e8f78236843861c7dd2ae22d