Misc Diffusion/Differential CSS tweaks
Summary: Various tweaks and fixes. Adds a File Contents view in Diffusion, normalizes spaces, colors.
Test Plan: tested differential and diffusion in my sandbox.
Reviewers: epriestley, btrahan
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Maniphest Tasks: T3952
Differential Revision: https://secure.phabricator.com/D7325