Make drag-and-drop on workboards interact with priority column headers
As of D20247, headers show up but they aren't really interactive. Now, you can drag cards directly underneath a header (instead of only between other cards). For example, if a column has only one "Wishlist" task, you may drag it under the "High", "Normal", or "Low" priority headers to select a specific priority.
(Some of this code still feels a little rough, but I think it will generalize once other types of sorting are available.)
Test Plan: Dragged cards within and between priority groups, saw appropriate priority edits applied in every case I could come up with.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20248