Implement viewing versions and downloading patches in Phragment

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