R1:b8551bb5f98c

Reduce drag-and-drop jank on dashboards

Summary:
Depends on D20414. Ref T13272. Several minor things here:

- Currently, you can drag panels underneath the invisible "there are no items in this column" div and the "Create Panel / Add Existing Panel" buttons. This is silly; stop it.
- Currently, when viewing a tab panel on a dashboard, you can drag the panels inside it. This is extremely silly. Make "movable" off by default and pass it through the async flow only when we actually need it.
- Make the…