Reduce surface area of DifferentialComment API

Summary:
Ref T2222. Shrink the API to make it easier to move this object's storage to ApplicationTransactions.

Fixes T3415. This moves the "Summary" and "Test Plan" into the property list, and thereby fixes all the attribution problems associated with commandeering, creating a revision from another user's diff, etc.

Test Plan: Browsed several revisions.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3415, T2222

Differential Revision: https://secure.phabricator.com/D7375