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.)
I tried using `safely_redirect_to_start_point` in the initial thing, but I couldn't work out how to reference it from the preview bulk add controller method, given it is is inside the mappings controller.