R1:c4ed47929c21

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…