Adds a new 'setnobackground' panel class, implements in Differential / Diffusion.

Summary: Still working through basic re-design. Adds the ability to re-use panel view without the background.

Test Plan: Viewed Diffusion and Differential in Chrome, FF, Safari.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4430