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 substantially separates the Maniphest-specific behaviors from the general dragging behaviors.

This diff causes no functional changes.

Test Plan: Dragged and dropped tasks in Maniphest.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6124