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 it's not redundant with other UI.

Test Plan: {F101208}

Reviewers: chad, btrahan

Reviewed By: chad

CC: chad, aran

Maniphest Tasks: T1344

Differential Revision: https://secure.phabricator.com/D7942