Create ChangesListSpecialPage as a base class for Watchlist and RC

Most of the code has yet to be actually moved there, but it's a start!
It's full of @todo comments, I'm going to go through them later.

This should introduce no functional changes and no backwards
incompatibilities. Verified that all special pages being subclasses
work as they did before (SpecialRecentChanges, SpecialWatchlist,
SpecialRecentChangesLinked).

Change-Id: Icb7671e92a9255619e047ccbe5f457aa22581479
Merge "Create ChangesListSpecialPage as a base class for Watchlist and RC"