Fix Differental Filetree browser hotkey
Summary: Adds a class for explicitly hiding the sidenav.
Test Plan: Set Config to Enable Filetree. View a diff, see tree. Press f, see it go away. Reload page, see persistence.
Reviewers: avivey, epriestley
Reviewed By: epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D16359