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 subpriorities.

Test Plan:

• Wrote and executed unit tests.
• Dragged and dropped tasks within priorities and between priorities in the main Maniphest view.
• Dragged and dropped tasks within priorities in the workboard view, when ordered by priority.
• Also poked at the "natural" order, but that shouldn't be affected.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: chad, epriestley

Maniphest Tasks: T5201, T7563

Differential Revision: https://secure.phabricator.com/D12121