code: def auth(self, request):
          
          service = UserService.objects.get(user=request.user, name='ServiceWallabag')
          callback_url = '%s://%s%s' % (request.scheme, request.get_host(), reverse('wallabag_callback'))
          params = {'username': service.username,
                    'password': service.password,
                    'client_id': service.client_id,
                    'client_secret': service.client_secret}
          access_token = Wall.get_token(host=service.host, **params)
          request.session['oauth_token'] = access_token
          return callback_url
docs: let's auth the user to the Service
              :param request: request object
              :return: callback url
              :rtype: string that contains the url to redirect after auth