Make the top nav non-fixed-position

Summary:
See some discussion in D1673.

• There's a concrete (if minor) problem with this in Firefox with wrapping search.
• People complain about how we're stealing all their pixels.
• There isn't much of a functional purpose to it since all the operations are fairly rare.
• This addresses the aesthetic purpose of the fixed-position nav (not making the side nav ugly) by making the side nav scroll up 44px and then stop.

Test Plan: Scrolled in desktop, tablet modes.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3412