Restore the "buoyant" header in Differential

Summary:
Fixes T1591. This was removed long ago because it was a mess to implement and caused a bunch of weird issues, and also my tolerance for dealing with weird JS issues was much, much lower.

I have now survived the fires of JX.Scrollbar and would love to address 200 small nitpicks about obscure browser behaviors on Linux, so open the floodgates again.

A secondary goal here is to create room to add a global view state menu on the right, with 300 options like "hide all inlines", "hide done inlines", "hide collapsed inlines", "hide ghosts", "show ghosts", "enable filetree", "disable filetree", etc, etc. Not sure how much of this I'll actually do. I have one more experiment I want to try first.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T1591

Differential Revision: https://secure.phabricator.com/D17945