Shift footer when on page with sidenav

Summary: Fixes T6168, adds spacing when navigation is present.

Test Plan: Tested Chrome, IE 9, 10. Numerous pages.

Reviewers: epriestley, btrahan

Reviewed By: btrahan

Subscribers: epriestley, Korvin

Maniphest Tasks: T6168

Differential Revision: https://secure.phabricator.com/D10543