Support "Test Plan" with modular transactions and EditEngine

Summary: Ref T11114. The only real trick here is that we respect configuration in differential.fields.

Test Plan: Turned plan on and off, tried to remove the plan, edited the plan.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17048