Make all CSS rules for draggable cards/items independent of container classes

Summary:
Ref T5240. With the new approach, the draggable clones lose their containers, so they don't get affected by rules like .container .item.

Put classes on the cards/items and use .board-item.item and .standard-item.item to apply rules instead.

This didn't turn out too gross, and seems relatively OK / not obviously broken.

Test Plan:

• Dragged cards on a workboard.
• Dragged items in normal lists (tasks, pinned apps).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5240

Differential Revision: https://secure.phabricator.com/D15161