R1:40af472ff595

Make drag-and-drop on workboards interact with priority column headers

Summary:
Ref T10333. Ref T8135. Depends on D20247. Allow users to drag-and-drop cards on a priority-sorted workboard under headers, even if the header has no other cards.

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"…