Rename ambiguous association

This was causing conflict with the association of the same name
on the Policy model when trying to mix in Edition::WorldwidePriorities
Can associate policies with worldwide priorities
List related documents for worldwide priorities

Includes some re-jigging of the CSS to allow the activity navigation
to be shared between priorities and publications.
Merge pull request #347 from alphagov/activity-for-priorities

Related docs activity for worldwide priorities
https://www.pivotaltracker.com/story/show/45719679