R1:5aca52998058

Fix literally thousands of drag-to-reorder priority bugs

Summary:
Fixes T7563. Fixes T5201. Reframe this as two separate operations:

- Move before or after a task.
- Move to the beginning or end of a priority.

Then:

- Make all the order queries unambiguous and properly reversible, with an explicit `id` order.
- Just reuse `ManiphestTask` to get results in the correct order.
- Simplify the actual transaction apply logic.
- Detect and recover from cases where tasks have identical or similar…