Add permission checking to revision diff

* Throw exception if users don't have permission view either revisions

* The diff should always run against wikitext

The way it works now is a little bit awkward, it will compare moderated html content if the content is not viewable. 

Change-Id: Ic34b168ea7f0126311329b042b1a9e522bc147ff
Merge "Add permission checking to revision diff"