R1:efcca310c2cf

In Differential mail, hide "summary" if empty and show "test plan" if nonempty

Summary:
Ref T4481. Summary is optional, but we currently always render it.

We previously rendered TEST PLAN. I wanted to see if anyone missed it. I miss it a little bit, and it sounds like @spicyj misses it. Restore it.

Test Plan:
$ ./bin/mail show-outbound --id 15232
...
BODY
epriestley created this revision.
epriestley added reviewers: The Bureaucracy, duck.
epriestley added a subscriber: duck.

TEST PLAN…