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