New hook accommodates non-revision rc queries

Within Special:RecentChanges the default non-extended query contains

    ( rc_this_oldid=page_latest OR rc_type=3 )

Wikidata has previously used the SpecialWatchlistQuery hook to look for
this exact string and change rc_type=3 to rc_type IN (3,5).  Flow is another
type of change that doesn't have a matching row in revisions to match
page_latest for and needs to be added to this query.

This patch adds a new hook, SpecialWatchlistGetNonRevisionTypes, which allows
different extensions to add to a list of values for rc_type (or rc_source once
85787 is merged).  This will allow multiple extensions to affect the resulting
query without them breaking eachother.

Change-Id: Id6916fe999c0faa38de878b7b5687e7ea00901bd
Merge "New hook accommodates non-revision rc queries"