R1:14a433c77355

Add priority group headers to workboard columns (display only)

Summary:
Ref T10333. When workboards are ordered (for example, by priority), add headers to the various groups. Major goals are:

- Allow users to drag-and-drop to set values that no cards currently have: for example, you can change a card priority to "normal" by dragging it under the "normal" header, even if no other cards in the column are currently "Normal".
- Make future orderings more useful, particularly "order by assignee". We don't…