Depends on D20321. Fixes T12175. Ref T13074. Now that before/after PHIDs are suggestions, we can give the server a more complete view of what the client is trying to do so we're more likely to get a good outcome if the client view is out of date.

Instead of passing only the one directly adjacent card PHID, pass all the card PHIDs that the client thinks are in the same group.

(For gigantic columns with tens of thousands of tasks this might need some tweaking -- like, slice both lists down to 10 items -- but we can cross that bridge when we come to it.)

• Dragged some cards around to top/bottom/middle positions, saw good positioning in all cases.
• In two windows, dragged stuff around on the same board. At least at first glance, conflicting simultaneous edits seemed to do reasonable things.