Add wgRevisionId variable to ResourceLoader

mw.config provides wgCurRevisionId, which has the latest
revision number. This adds wgRevisionId, which indicates
the revision number currently being viewed. This is useful
when looking at old revisions.

Bug: 51594
Change-Id: Ie0d00f3a9a8af8533ab28204b5bb483a0092c710
Merge "Add wgRevisionId variable to ResourceLoader"