Testing for prod mode

[Sim4n6]

Idea

• [x] made a unit test for production mode, not debug mode nor testing mode.
• [x] need to set those flags in config correctly.
• [x] set also a secret key in the appropriate location 👍
• [x] check deployment checklist

Reference

• https://devcenter.heroku.com/articles/getting-started-with-python
• https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-xviii-deployment-on-heroku
• https://flask.palletsprojects.com/en/1.1.x/config/