Implement "Test Plan" as a CustomField
Summary: Ref T3886. Moves some of the "required" logic to the base class ("DifferentialCoreField") so Title and Test Plan can share it.
Test Plan: Edited revisions using "pro" editor, saw test plan transactions.
Reviewed By: btrahan
Maniphest Tasks: T3886
Differential Revision: https://secure.phabricator.com/D8285