Add hotkey for hiding file tree

Summary:
In Differential, viewer can hit 'f' key to hide/show file tree on the left
side. Useful on narrow monitors.

Test Plan: Open any diff in Differential tool, hit 'f', watch file tree disappears

Reviewers: vrana, mattchoi, epriestley

Reviewed By: vrana

CC: aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D3844