Display Reviewed By instead of first Reviewer in revisions overview

Summary:
Displaying reviewer who was by coincidence listed first is quite confusing, especially for committed revisions.
This displays the one who really reviewed the revision if available.

This implementation is pretty bad from performance perspective - O(N) queries to retrieve all comments.
The page load still feels quite fast.

Test Plan: /differential/filter/revisions/

Reviewers: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1772