Shorten the "Successfully importing a large batch of mappings" feature

- This still keeps some of the old steps in `steps %{}` blocks within
  the new more readable and concise ones. Not sure if this is what
  Russell meant at all, but it still works.
Shorten the "Successfully importing a small batch of mappings" feature

- This makes what the behaviour is clearer than the implementation of
  said behaviour.
- This still keeps some of the old steps in `steps %{}` blocks within
  the new more readable and concise ones. Not sure if this is what
  Russell meant at all, but it still works.
Move a step only used in one feature into a feature helper
Merge pull request #358 from alphagov/readable_import_mappings_features

Improve readability of import mappings features