Add a "View Revision" button to HTML email

Summary:
Ref T10694. If this feels good, I'd plan to eventually add something similar to other applications ("View Task", etc).

Not sure if we should keep the object link later in the mail body or not. I left it for now.

Test Plan:

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10694

Differential Revision: https://secure.phabricator.com/D15884