Show search/login on tablet and mobile.
Summary: We should always have some sort of menu on mobile for logging in.
Test Plan: Test mobile, tablet, and desktop breakpoints. Gate seearch icon by public_policy.
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T4731
Differential Revision: https://secure.phabricator.com/D8868