Define Differential email action in terms of EditEngine

Summary: Ref T11114. Move email/command actions, like "!reject", to modular transactions + editengine.

Test Plan: Used bin/mail receive-test to pipe "!stuff" to an object, saw appropraite effects in web UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17133