Mobile Differential Diff Review (2-up)

Summary:
This does two things

 - Modernizes Table of Contents
 - Makes Differential reasonable on mobile

I say resonable, as you still have to scroll horizontal to see the entire diff. This is minor as the rest of the page is 100x more useful. A 1-up view would be preferred, but this is still an improvement.

Test Plan: Used iOS simulator for browsing diffs.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin, chad

Differential Revision: https://secure.phabricator.com/D8681