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 around OK

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5240

Differential Revision: https://secure.phabricator.com/D10188