R1:5f98c5f386d9

Simplify drag-and-drop list implementations

Summary:
- Use the same styles for shared operations (`drag-ghost`, `drag-dragging`).
- Move shared code into the base class.

Test Plan: Dragged around tasks and named queries.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6141