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.

Test Plan: On a workboard with a mixture of editable and not-editable cards, hovered over the different cards and was able to figure out which ones I could drag or not drag pretty easily. Picked up cards I could pick up, wasn't able to drag cards I can't edit.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T10722

Differential Revision: https://secure.phabricator.com/D20274