Make collapsed navigation sticky

Summary: This is the most requested feature in FB by far.

Test Plan:
Toggled, verified that data are saved.
Reloaded, toggled, toggled, toggled, verified that data are saved.
Reloaded.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4812