R1:00543f0620d1

Remove the ability to drag tasks up and down on (non-Workboard) priority list views

Summary:
Ref T13074. Today, in normal task list views in Maniphest (not workboards), you can (sometimes) reorder tasks if the view is priority-sorted.

I suspect no one ever does this, few users know it's supported, and that it was basically rendered obsolete the day we shipped workboards.

This also means that we need to maintain a global "subpriority" for tasks, which distinguishes between different tasks at the same…