R1:476b7ef81611

Redesign main header

Summary: First cut of redesign branch, simpler, lighter header. Will check in new graphics in follow up diff.

Test Plan: Mostly all just new colors, but this also removes setting of `header-color` in Config.

Reviewers: epriestley, btrahan

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12745