add login blueprint

[WolfgangFahl]

refactor login functionality of example app to a blueprint

see

• https://flask.palletsprojects.com/en/1.1.x/blueprints/
• https://www.digitalocean.com/community/tutorials/how-to-add-authentication-to-your-app-with-flask-login-de
• https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-i-hello-world
• https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-v-user-logins

[WolfgangFahl]

• https://stackoverflow.com/questions/47636610/flask-sqlalchemy-mssqlpyodbc-and-existing-models-with-blueprint issue comes up ...
• https://stackoverflow.com/questions/60184852/removing-gloabl-app-reference-from-blueprint-in-flask
• https://stackoverflow.com/questions/55782492/creating-flask-sqlalchemy-instance-with-metadata-from-blueprint?rq=1
• https://stackoverflow.com/questions/53223826/adding-a-flask-class-instance-into-a-loginmanager-class-instance
• https://stackoverflow.com/questions/65225026/how-do-i-create-a-flaks-sql-alchemy-db-in-a-flask-blueprint

[WolfgangFahl]

Ask for improvement upstream see https://github.com/pallets/flask-sqlalchemy/issues/903