Add a Cancel button to the bulk add preview page

- This uses the OffSiteRedirectChecker and params[:return_path] to
  return the user to where they were before they arrived on the bulk
  add page, so long as it was on the site, otherwise they get returned
  to the site mappings page.
Preserve query parameters for the return_path when adding mappings
Merge pull request #410 from alphagov/bulk_add_preview_cancel_button

Add a Cancel button to the bulk add preview page