Make priorities optional in the issue tracker UI (BB-8871)

Some people may have workflows that don't use priorities, so it would be nice if they were optional.

In my workflows, priorities are just another way of communicating in what order something should be done. And since this is already exposed in the UI through milestones, why have both? I'd like not to have to specify one when I don't use it.

Also see http://www.43folders.com/2009/04/28/priorities