Add Ajax notifications API

[Abdur-rahmaanJ]

[rehmanis]

Is this for replacing backend flask notification with frontend? Our app is mostly server-side but would be good to add client-side ajax calls depending on the case for example not having to reload the url to update data etc.

[Abdur-rahmaanJ]

@rehmanis no new features. Just a mechanism to add notification as described here: https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-xxi-user-notifications