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
38739755c6