R1:e433a09fde9d

Scroll parent containers when objects are dragged near the edge of the field of view

Summary:
Ref T5240. This probably has some bugs and doesn't quite work in Firefox (fine on boards, not quite on the task list -- some issue with body or document being special, I think).

I think this is close enough that we can throw it out there and see how users manage to break it, though. It's not worse than what we've got now? I think?

Test Plan:
dragged things near the edge of other things

they seemed to move…