Ref T5476. Currently, the task edit code assumes it knows what the UI looks like and sends back where on the column an item should be inserted.

This is buggy after adding filters, and relatively complex. Instead, send down the ordering on the whole column and sort it in the UI. This is a bit simpler overall and more general. It makes it easier to further generalize this code for T5476.

• Edited a task on a board, changing priority. Saw it reorder properly.
• Edited a task on a board in a field of other tasks at the same top-level priority. Saw it refresh without reordering.