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 relevant operations just apply to the whole group of lists.

Test Plan:

• Verified existing funtionality in Maniphest and ApplicationSearch is unaffected, by dragging around tasks to reprioritize them and dragging around search items.
• Dragged tasks between columns on a board view.

{F101196}

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T1344

Differential Revision: https://secure.phabricator.com/D7941