R1:fb2da8bd8b43

Add links and diffs for text block edits to mail

Summary:
Ref T7643.

- When a transaction edits a text block, add a link to the changes (for HTML mail).
- Also, inline the changes in the mail (for HTML mail).
- Do nothing for text mail since I don't think we really have room? And I don't know how we can make the diff look any good.

Test Plan:
Edited a task description, generated mail, examined mail.

- It contained a link leading to a prose diff.
- It had a more-or-less reasonable inline text…