Use FontAwesome for search icon

Summary: Swaps out search icon for font-awesome. Tweaks colors a bit for different color headers.

Test Plan: Test a number of colors in the search box. Click on button and still go to advanced search.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12200