Differential - finesse Differential diff view controller

Summary:
Fixes T7229. Some usability issues around this controller - basically you can't leave comments with it and its not particular useful compared to the revision page.

Ergo, if there is a revision associated with a given diff, just re-direct back to the revision page with the proper diff loaded.

Test Plan: Tried to view a diff on the standalone controller attached to a revision and instead was re-directed to the revision view page with the proper diff loaded.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7229

Differential Revision: https://secure.phabricator.com/D11811