Use prose diffs in Phriction

Summary: Ref T3353. This improves whitespace display of prose diffs and uses them in Phriction.

Test Plan: {F1677957}

Reviewers: chad, avivey

Reviewed By: avivey

Subscribers: avivey

Maniphest Tasks: T3353

Differential Revision: https://secure.phabricator.com/D16062