Highlight reviews the viewer is responsible for in Differential

Summary: Ref T1279. No logical changes, but cosmetically highlight stuff you have authority for, like we do in Diffusion.

Test Plan: See screenshot.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7237