adding notification event to index.js under "crowi" directory

packages/app/src/server/crowi/index.js

@@ -80,6 +80,7 @@ function Crowi() {
     bookmark: new (require('../events/bookmark'))(this),
     tag: new (require('../events/tag'))(this),
     admin: new (require('../events/admin'))(this),
+    notification: new (require('../events/notification'))(this),
   };
 }