Add a "Comment content" field to Herald

Summary: Ref T13583. To improve support for making it harder to improperly mix data retention policies, allow Herald to act on comment content.

Test Plan:

• Wrote comment content Herald rules in Maniphest and Differential.
• Submitted non-matching comments (no action) and matching comments (Herald action).
• In Differential, triggered rules by submitting non-matching main content and a matching inline comment.

Maniphest Tasks: T13583

Differential Revision: https://secure.phabricator.com/D21479