Workboards - add task create + improve task placement wrt priority edits

Summary: Fixes T4553, T4407.

Test Plan: created tasks and they showed up in the proper column. edited task priority and they moved about sensically.

Reviewers: chad, epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4553, T4407

Differential Revision: https://secure.phabricator.com/D8420