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 swapping the priorities of two
tasks ~50 times. This case is pretty silly and pathological. We can add some
code to deal with this at some point if necessary.

I think this also fixes the whacky task layout widths once and for all.

(There are a couple of minor UI glitches like headers not vanishing and header
counts not updating that I'm not fixing because I am lazy.)

Test Plan: Dragged and dropped tasks around.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley, mgummelt

Maniphest Tasks: T859

Differential Revision: https://secure.phabricator.com/D1731