Restore "Accept", "Reject" and "Resign" actions to Differential on EditEngine

Summary:
Ref T11114. Some rough edges, but this largely makes Accept, Reject and Resign work in the new EditEngine comment area.

Ref T11050. This lays a little bit of groundwork for having "resign" mean "I don't want to review this, even if projects or packages I'm a member of need to", not just "remove me personally as a user reviewer".

Test Plan: Accepted, rejected and resigned from revisions without any major state issues.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114, T11050

Differential Revision: https://secure.phabricator.com/D17113