code: def activate(request, activation_key=None):
      
      utils.activate_user(activation_key)
      # if not activated
      success_url = utils.get_settings('REGISTRATION_API_ACTIVATION_SUCCESS_URL')
      if success_url is not None:
          return HttpResponseRedirect(success_url)
docs: Given an an activation key, look up and activate the user
      account corresponding to that key (if possible).