R1:75a04551524f

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.