Edition worldwide priorities relate to document

Previously the worldwide priorities were joining to edition ids, which is incorrect: if the document is subsequently republished, then we won't be pointing to the right edition. This change appropriates the existing join for policies and uses it for priorities also.
    
When associating policies and priorities to the same edition we previously had clashes. Turns out it's not actually possibly yet to associate policies and worldwide priorities, but this may come up in the future and it's good to have a central place that we record all edition -> document relations that works as advertised rather than inconsistent behaviour.
    
Ticket: https://www.pivotaltracker.com/story/show/45829847