Improve some search behaviors on mobile/devices

Summary:
Ref T2625.

  - Build the mobile menu from the delegating controller.
  - Make the result header look a little better (still a bit funky).

Test Plan:
{F44774}

{F44775}

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6090