ChangesListSpecialPage: Separate all functionality for generating feeds

I should have done it at the beginning instead of trying to
extract it from recent changes.

Same for SpecialRecentChanges and SpecialRecentChangesLinked
(subclasses).

Created a new API module for it: ApiFeedRecentChanges.
It's somewhat un-API-like and hackish, but all feed modules are.

Old URLs redirect to new ones, so this should be fully
backwards-compatible assuming sane feed reader clients.

Change-Id: I06ee0f01d896bc66545a1800b24693ce7524e433