Fix topical event atom feeds

Topical event atom feeds were broken due to `app/views/classifications/show.atom.builder` calling `topic_url(@topic)`. I have updated it to use a generic `classification_url` method and made both the `topics_controller` and `topical_events_controller` set a `@classification`. 

I also took the liberty of moving the topic view into the topics folder as its not a generic `classification` view, and introduced/removed some partials.