Adjust steps for topics and topical events

No need to drive the UI to create fixture data in setup steps. Also
move the helper methods out of the steps files.
Failing acceptance test
Simplify classication form templates
Rework functional tests and add missing coverage
Fix broken markup
Add admin::topics#show page

Ready for a tabified interface for managing features
Move classification featuring code to base class
Add featuring to topics administration
Improve UI and wording for featuring documents

* Only show the sorting controls if there are more than one
* Add the "changes are instant" warning
* Mention that a maximum of five documents will be displayed
* Change sort order button text
Stop sort-order text field flashing up
GET rid of n+1 queries on topic featuring page

admin/classification_featurings#index was making 1.6k queries for
the 'Arts and culture' topic. This reduces it down to less than 10.