Summary:
This adds support for viewing individual versions on a fragment as well as comparing versions and downloading diff_match_patch-based patches.
It does not use the side-by-side diff format as while it works for small changes, it quickly becomes impossible to distingush what changes have been made due to the diff_match_patch format.
Test Plan: Clicked on versions and downloaded patches.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Maniphest Tasks: T4205
Differential Revision: https://secure.phabricator.com/D7734
