Fix two issues with embedding other fields inside "Summary" or "Test Plan" in Differential with the web UI

Summary:
Ref T11114. Converting to EditEngine caused us to stop running this validation, since these fields no longer subclass this parent. Restore the validation.

Also, make sure we check the first line of the value, too. After the change to make "Tests: xyz" a valid title, you could write silly summaries / test plans and escape the check if the first line was bogus.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17248