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 zany priority changes (in "natural" mode).
• Created new tasks, they show up at the top.
• Tagged new tasks, they show up at the top of backlog.
• Swapped to "priority" mode and got sorting and the old priority-altering reordering.
• Added tasks in priority mode.
• Viewed task transactions for correctness/sanity.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: chad, epriestley

Maniphest Tasks: T4807

Differential Revision: https://secure.phabricator.com/D10182