Remove header gradients for flat colors

Summary: Removes header gradient images for flat, CSS controlled colors. I didn't convert the "pony" colors over, going with few options for easier theme-ability.

Test Plan:
Test each color choice.

{F1063828}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15052