Support (basic) commenting on Phriction documents
- Add an EditEngine, although it currently supports no fields.
- Add (basic, top-level-only) commenting (we already had the table in the database).
This will probably create some issues. I'm most concerned about documents accumulating a ton of old, irrelevant comments over time which are hard to keep track of and no longer relevant. But I think this is probably a step forward in almost all cases, and a good thing on the balance.
This also moves us incrementally toward putting all editing on top of EditEngine.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D19660