R1:ffb7978528e4

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…