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 diff.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16063