R1:826914e9901a

Allow tasks to be dragged-and-dropped between workboard columns (UI only)

Summary:
Ref T1344. Allows you to drag tasks within a column and between columns, and handles all the multi-column state / targeting / ghosting stuff.

This is a UI-only change; you can't actually do anything meaningful with these yet.

Roughly, I added the idea of a DraggableList existing within a "group" of draggable lists. Normally, that group only has one item, but on boards it has all of the columns. Then I made all of the…