Implement full support for enhanced changes format in client recent changes and watchlist.
This requires a bit of refactoring and cleanup of spaghetti code in core, and then simplify / provide a more fine-grained way for extensions to hook into the core code that generates the recent changes lists in the special pages.
This should be easier now that rMW78f109bfddd0: Generalize recentChangesFlags rollup is in.