Displaying links instead of dumping raw remarkup in Phriction mails

Summary:
Fixes T2900 - Display link to document version and diff for edited Phriction Documents.

Also display the target document for moved pages.

Test Plan: Edited/Moved some pages, verified that the links are linking to the correct versions.

Reviewers: epriestley

Reviewed By: epriestley

CC: adityar7, aran, Korvin

Maniphest Tasks: T2900

Differential Revision: https://secure.phabricator.com/D5615