Fix polymorphic association for contexts
-rename has many assoc. to be more readable
-a context has many sub_topics in the same way a topic as many sub_topics
Fix naming error on topicable_topics table
-belongs to topic table not "topics" table
Fix and make topic associations more readable
-fix misnamed polymorphic associations
-distinguish which way the association is going (topic has-many topics!) by aliasing the join table for one direction. Called sub_topic
-a topic has many sub_topics
-a topic has many topics if it itself is a subtopic of those topics
-a topic has many contexts if it itself is a subtopic of those contexts