Improve readability of import mappings features

- Shorten the "Successfully importing a small batch of mappings" feature. 
- Shorten the "Successfully importing a large batch of mappings" feature.
- Both of these still keep 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 in https://github.com/alphagov/transition/pull/334#discussion_r14513210 and below, but they're still green and they now seem to document the behaviour more than the implementation.