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