R1:804be81f5dbb

Provide better UI feedback about cards that can't be dragged or edited

Summary:
Depends on D20273. Fixes T10722. Currently, we don't make it very clear when a card can't be edited. Long ago, some code made a weak attempt to do this (by hiding the "grip" on the card), but later UI changes hid the "grip" unconditionally so that mooted things.

Instead:

- Replace the edit pencil with a red lock.
- Provide cursor hints for grabbable / not grabbable.
- Don't let users pick up cards they can't edit.