Restore "Plan Changes" and "Request Review" actions to Differential on EditEngine

Summary:
Ref T11114. This restores these actions.

One behavior is incomplete: "Request Review" on an accepted revision does not downgrade reviewers properly. I've noted this locally.

Test Plan: Planned changes and requested review of a revision.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17109