differntial and diffusion design tweaks

Summary: most awesome is that differential-primary-pane no longer has a place in diffusion. less awesome is fixing the zebra striping on differential "Local Commits" view and making the font size of one of the table headers match the others.

Test Plan: looks good!

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4177