Add ParserPreSaveTransformComplete hook

This is needed to migrate an extension (AutoWIGO2) that uses ArticleSave
to a version of MW after that hook was removed. The extension was using
ArticleSave to do its own PST processing, which probably doesn't work
with PageContentSave, and wasn't really the right way to do it in the
first place. Surprisingly, there were no useful hooks in PST or its
callers.

Change-Id: If1b3d7ea130a2b77609afe206eedc8445e1ab9f9