Store "last comment" and "last action" diffs on reviewers

Summary:
Ref T10967. We have a "commented" state to help reviewers get a better sense of who is part of a discussion, and a "last action" state to help distinguish between "accept" and "accepted an older version", for the purposes of sticky accepts and as a UI hint.

Currently, these are first-class states, partly beacuse we were somewhat limited in what we could do with edges. However, a more flexible way to represent them is as flags separate from the primary state flag.

In the new storage, write them as separate state information: lastActionDiffPHID stores the Diff PHID of the last review action (accept, reject, etc). lastCommentDiffPHID stores the Diff PHID of the last comment (top-level or inline).

Test Plan: Applied storage changes, commented and acted on a revision. Saw appropriate state reflected in the database.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10967

Differential Revision: https://secure.phabricator.com/D17514