Show change diffs in Phriction

Summary:
This is really rough and needs work (particularly, there's some diff code I
really need to refactor since I sort-of-copy-pasted it) but basically
functional.

Show text changes between diffs and allow users to revert to earlier versions.

Differential's line-oriented diff style isn't ideal for large blocks of text but
I'm betting this is probably good enough in most cases. We can see how bad it is
in practice and then fix it if needbe.

I added a bunch of support for "description" but didn't add the feature in this
diff, I'll either follow up or task it out since it should be a pretty
straightforward change.

Test Plan: Looked at history for several Phriction documents, clicked "previous
change" / "next change", clicked revert buttons.
Reviewed By: hsb
Reviewers: hsb, codeblock, jungejason, tuomaspelkonen
CC: aran, hsb, epriestley
Differential Revision: 687