code: def callback(self, request, **kwargs):
          
          if self.oauth == 'oauth1':
              token = self.callback_oauth1(request, **kwargs)
          else:
              token = self.callback_oauth2(request)
  
          service_name = ServicesActivated.objects.get(name=self.service)
  
          UserService.objects.filter(user=request.user, name=service_name).update(token=token)
          back = self.service.split('Service')[1].lower()
          back_to = '{back_to}/callback.html'.format(back_to=back)
          return back_to
docs: Called from the Service when the user accept to activate it
              the url to go back after the external service call
              :param request: contains the current session
              :param kwargs: keyword args
              :type request: dict
              :type kwargs: dict
              :rtype: string