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.
Update topics#show to list featured documents
Simplify tab_navigation_for helper
Don't redirect to index on create or update
Remove pointless instance variable
DRY up classifications#index
DRY up classifications#show
Only show related topics if there are some
Consistent naming of factory files

Otherwise they're hard to find
Add separate action for re-ordering features

The way it was done previously using the classifications#update action
meant that any related classifications were being cleared every time
the features were-reordered. This mirrors what we already do for
re-ordering attachments.

I also updated the factory for a classification featuring to ensure
the featured edition is published, otherwise the association on
classification does not return them as it has an explicit conditiion
set to only load those with published editions