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.
- 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.