Add Special:GloballyWantedFiles

Like Special:WantedFiles, but for all wikis.

Some functionality was added as static methods to GlobalUsage
in order so that other special pages could share the code.

To fully work, this needs Ic72be60d13, but it degrades gracefully
without that change.

Change-Id: I91ce050d46096326c2b6508a34a30b71695cb21e