R1:a101b4ba2ed2

Make the rest of the board drag-and-drop UI mostly work

Summary:
Ref T1344. Makes the UI/UX a little nicer; still no actual backend stuff. This changes:

- When you drop an item onto a different column, the item actually moves.
- Empty columns render with a special CSS class now, but no nodes in the list. This cleans up some JS jankiness. I made the "empty" columns have a light blue background for now. We could put some sort of subtle background image in them instead, or some kind of call to action if…