Allow multiple PT states per BP column

This is because a project that switched from automatic planning to manual planning suddenly saw all the unstarted column go empty.  Turns out the unstarted stories were in a new to BP state called "planned" and this meant they didn't appear in any column.  If we allow for multiple PT states per BP column then we can put both "unstarted" and "planned" PT state stories into the "unstarted" BP column.