Properly change device after resizing left nav

Summary: Also allow left nav to hide.

Test Plan:

1. Resize left nav.
2. Shrink browser width to switch device.
3. Increase the width again.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3383