Add a "Published document changed" rule to Herald for Phriction documents

Summary: Depends on D20519. Ref T13283. See PHI1202. Add a new rule which triggers when the current/most-recent transaction group includes a "content" or "publish" transaction, which means the published document content has changed.

Test Plan:

• Wrote a Herald rule using this field.
• Created a document (rule matched).
• Edited a document (rule matched).
• Edited a document, saving as a draft (no match).
• Edited a draft, updating it (no match).
• Published a draft docuemnt (rule matched).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13283

Differential Revision: https://secure.phabricator.com/D20520