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.

(Thanks to @rgarner for helping with testing this.)
This redirect isn't preserving query params for filtered mappings. I think it would need to do something [like this](https://github.com/alphagov/transition/blob/master/app/controllers/mappings_controller.rb#L179) for that to work - the redirect after saving a single edited mapping does preserve the filter.