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.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3886

Differential Revision: https://secure.phabricator.com/D8285