Add edition to policy topics when setting policies

Announcement and Publicationesque editions can be assigned to policies
and topics separately. When a policy is chosen (in the admin interface),
an AJAX request is made to determine the policy's topics. Those topics
are then automatically selected in the topics select tag.
Cucumber scenarios for assigning editions to topics
Tidy up TopicsController
Mercilessly refactor the incomprehensible
WIP: Return directly and indirectly associated docs when filtering by topic