When users scroll while dragging items, update item positions immediately

Summary:
Fixes T5979. When you drag a task and scroll using the mouse wheel, we don't move the task under your cursor until you move the mouse.

Instead, listen for `scroll` and move the task.

Test Plan:
  - Clicked and dragged a task.
  - While holding the task and not moving the cursor, used the mouse wheel to scroll.
  - The task followed the cursor (previously, it stayed in position until the mouse was moved again).

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5979

Differential Revision: https://secure.phabricator.com/D10416