Fix footer on mobile/nav pages

Summary: We should only move the footer over on desktops, where the sidenav is still present.

Test Plan: Test Mobile, Tablet, Desktop breakpoints on Herald.

Reviewers: epriestley, btrahan

Reviewed By: btrahan

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10609