Add storage for new Maniphest transactions

Summary: Ref T2217. Add the tables and comment class for the new stuff. Not used yet.

Test Plan: Ran storage upgrade, browsed Maniphest.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7066