Add "Revision test plan" as a Herald field; remove test plan from the "Revision summary" field

Summary:
See PHI844. Ref T13189.

Add "Revision test plan" as an available field for Herald. This is a little niche -- and a little odd because it sticks around even if you fully disable test plans -- but probably broadly reasonable.

The existing "Revision summary" field counterintuitively included the test plan. Separate this out since it's now a separate field and the behavior was weird historic nonsense. I'll note this in the changelog.

Test Plan: Wrote a rule using both fields, verified they generated the expected values.

Reviewers: amckinley

Maniphest Tasks: T13189

Differential Revision: https://secure.phabricator.com/D19623