Hide footer on mobile app switcher

Summary: Fixes T7609. When we moved the sidenav background to z-index -1, the footer then reappeared. Make the hiding more specific in CSS.

Test Plan: test mobile layout of homepage, check footer still exists on all other pages.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7609

Differential Revision: https://secure.phabricator.com/D12138