Render sometimes-legible prose diffs in the Phabricator UI

Summary:
Ref T3353. This hooks the prose engine up to the UI and throws away the hard-wrapping hacks.

These are likely still very rough in many cases, but are hopefully a big step forward from the old version in the vast majority of cases.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3353

Differential Revision: https://secure.phabricator.com/D16056