Support ApiPageSet in ApiWatch

Add support for ApiPageSet capabilities in ApiWatch to support batch
operation on titles, pageids etc. The old 'title' parameter (for a
single page) is still supported to keep backwards compatibility.

Moved a function from ApiPurge to ApiPageSet: getInvalidTitlesAndRevisions() gathers
warnings about invalid/missing titles/ids in the requested page set.

Split from If993f6e8.

Change-Id: I820dcb64d469616b10741df013911197cc5bde29
Merge "Support ApiPageSet in ApiWatch"