Add storage for new Differential transactions and transaction comments

Summary:
Ref T2222. Ref T1460. Depends on D6260.

This creates the new tables, but doesn't start using them. I added three new fields for {T1460}, to represent fixed/done/replied states.

Test Plan: Ran storage upgrade.

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T1460, T2222

Differential Revision: https://secure.phabricator.com/D6261