Only load mw.PopUpMediaTransform on pages that plausibly need it

Rather than load mw.PopUpMediaTransform unconditionally, rely on the parser
hook to cover most cases, and use TimedMediaHandlerHooks::pageOutputHook to
cover the rest: namely, special pages that are liable to include timed media.

Bug: 55550
Change-Id: I7a6da6c62ceae1dae2feb05b57b2df91b482ad8b