Don't show the "file tree" view on tablets/phones

Summary: Ref T13516. Hide this UI on devices without the screen width to reasonably support it.

Test Plan: Viewed a revision at various window widths, saw the elements vanish at device widths and reappear at desktop widths.

Maniphest Tasks: T13516

Differential Revision: https://secure.phabricator.com/D21162