Handle rows with just a separator in import

This was triggering an error when ImportBatchRow tried to call strip on the
first entry in a row which it assumes is a string.
Merge pull request #356 from alphagov/handle_rows_with_just_a_separator_in_import

Handle rows with just a separator in import
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.