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