SSP-3029 Fix Action Plan new-task drop behaviors

 - Support multi-referral drag-and-drop (previously only handled
   the first selection in a multi-drop - was just missing a loop)

 - Disable drag-and-drop from *within* the new-task drop target. I.e.
   can't try to reorder tasks anymore except indirectly by removing
   them and re-adding a different position. The drag-to-reorder
   didn't work (would just clobber the target). And it has no
   real effect anyway since the order is lost when you view the
   entire plan.