Specify the ordering of the bulk add preview "first 20 mappings"

- The bulk add preview page shows the first 20 mappings, but the
  ordering wasn't set so potentially mappings /a, /b, /c could have
  ended up as /a, /c, /b in the preview. This fixes that bug by
  specifying the ordering as in
  https://github.com/alphagov/transition/blob/postgres-master/app/presenters/batch_preview_presenter.rb#L23.

This is relevant only to PostgreSQL.