Restore "Commandeer" action to Differential on EditEngine

Summary:
Ref T11114. This has two pieces of side-effect logic which I've noted locally:

• Commandeer needs to apply Herald rules.
• Commandeer needs to move the old author to become a reviewer and remove the actor as a reviewer.

Test Plan: Commandeered some revisions.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17111