Store revision reviewer state as edges

Summary:
Keep track of the state of a reviewer in an edge between reviewer and revision.
The edge stores the state of the review, added or rejected. And if the revision was
accepted by that reviewer the id of the diff accepted.

Test Plan:
Create diffs and clowncopterize reviewer list changes. This includes:

• Adding new reviewers
• Resigning
• Commandering a revision

Reviewers: epriestley

CC: aran, Korvin

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D6372

Conflicts:
src/applications/differential/editor/DifferentialCommentEditor.php