[Redesign] Fix Maniphest subpriority editing

Summary: Ref T8099, Fixes T8338. This allows re-ordering of Maniphest Tasks in the redesign. Somehow seems more fragile, but I couldn't break anything with it.

Test Plan: Try ordering into first position after header, last position, changing priority outright, everything I can drag.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099, T8338

Differential Revision: https://secure.phabricator.com/D13487