Move pattern search into Diffusion header

Summary: This is only on browse pages, but I think could be global (home) also. Moves it from a button, field, to just a field.

Test Plan:
Review search on desktop, mobile.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18428