Improve display of "Added Reviewers" and "Added CCs" in Differential, link diffs

Summary:
When a comments add reviewers or CCs, we just dump that sort of nastily into the
body. Put it in the header like Maniphest instead.

Also, record the diff associated with "update" actions and link to it (T871).

Test Plan: {F8546} {F8547}

Reviewers: btrahan, davidreuss

Reviewed By: davidreuss

CC: aran, epriestley

Maniphest Tasks: T871

Differential Revision: https://secure.phabricator.com/D1659