Improve minor workboard drag behaviors

Summary:
Ref T5240.

• Add proper class when dropping cards.
• Add proper class when creating new cards.
• Make X-drag explicit so that it works if there's only one column.
• Stop tootips when dragging, resume them after dropping.
• Move CSS rule for consistency.
• Allow user to hit "Escape" to cancel an in-progress drag.

Test Plan:

• Dropped cards.
• Created new cards.
• X-dragged on a workboard with one column and a dashboard.
• Dragged over a tooltip (no tip), dropped, moused over tooltip (tip).
• Hit escape during a drag.

Reviewers: chad

Reviewed By: chad

Subscribers: cspeckmim

Maniphest Tasks: T5240

Differential Revision: https://secure.phabricator.com/D15163