R1:7fbfeca8027c

Mostly generalize Maniphest's drag-and-drop list

Summary:
I want to use draggable lists in at least three other interfaces:

- (Today) Reorganizing named search queries.
- (Today) Reorganizing custom fields.
- (Future) Dragging tasks around on boards.

This mostly generalizes the drag-and-drop code in Maniphest's task list. It isn't a total generalization and will need some more tweaking (for example, Maniphest's list is unusual in that the user can't drag items to the top of the list), but it…