notifyUser(room, request, scope, content) {
    const notification = UserNotification.fromRequest(request, scope, content);

    global.kuzzle.emit("core:notify:user", {
      notification,
      room,
    });

    return this._notifyUser(room, notification, {
      fromCluster: false,
    });
  }
Broadcast a notification about a user entering or leaving
the provided room

@param {string} room - Room entered or left
@param {Request} request - User (un)subscription request
@param {string} scope - 'in' or 'out'
@param {object} content - Notification additional informations

@returns {Promise}