Support (basic) commenting on Phriction documents

Summary:
Depends on D19659. Fixes T1894. Ref T13077. See PHI840.

• 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.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13077, T1894

Differential Revision: https://secure.phabricator.com/D19660