Workboards - make priority changes less aggressive and generally better

Summary: Fixes T4641.

Test Plan: Dragged a "normal" task between "high" and "low" tasks and it stayed as "normal". Generally seems correct when playing around.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: mbishopim3, Beltran-rubo, epriestley, Korvin

Maniphest Tasks: T4641

Differential Revision: https://secure.phabricator.com/D8622