R1:59a85e8845ef

Support natural ordering of workboards

Summary:
Ref T4807. This is probably a complete fix, but I'd be surprised if there isn't a little cleanup I missed.

When users drag tasks on a "natural"-ordered workboard, leave things where they put them.

This isn't //too// bad since a lot of the existing work is completely reusable (e.g., we don't need any new JS).

Test Plan:
- Dragged a bunch of stuff around, it stayed where I put it after dropped and when reloaded.
- Dragged stuff across priorities, no…