Remove unreferenced steps and add helpers

 * Remove steps not mentioned in any feature
 * Move steps only called from other steps into a module
   (quite generic at this point)
Stop describing implementation in the feature step text
Move matchers and helpers to their right place

* Helpers are modules mixed into the Cucumber world and are a preferred unit of reuse
  when you might otherwise have to refer to another step that isn't used in a feature.
* Matchers are just RSpec matchers
Merge pull request #249 from alphagov/cuke-cleanup

Cuke cleanup
Add a basic feature to sort mappings by priority

- made-up numbers in the table, all the same
- not actually doing any sorting
- sort option only shows up on the non-js filter page
Remove extraneous space
Add a 'scopes' block in mapping_spec

* Move the only other scope tests to it
Add a with_traffic_summary scope to Mapping

* has counts (no percentages)