Make workboards truly 'fixed' in display

Summary:
**WIP**

This sets a shadow fixed to the full screen of the browser for which panels to live in. I have some minor things to fix, but play with it.

Specifically I haven't been able to dig up when stuff that overflowwed, doesn't always render even after scroll.

Test Plan: Test on a board with many tasks, many panels, small screens. Test on mobile too!

Reviewers: epriestley, btrahan

Reviewed By: btrahan

Subscribers: qgil, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9005