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