R1:e7853e48013e

Allow tasks to be subprioritized by drag-and-drop

Summary:
Like the title says, similar to Facebook Tasks.

Not sure how I really feel about this, but I guess it's kind of OK? I never used
this feature in Facebook Tasks but I think some people like it.

The drag-and-drop to repri across priorities feels okayish.

Because subpriority is a double and we just split the difference when
reprioritizing, you lose ~a bit of precision every time you repri two tasks
against each other and so you can break it by…