Refactor the website backend to a python `flask` backend

[ctmbl]

Here is a non-exhaustive list of things we must implement:
(heavily based on the current expressJS backend, some things might become useless or implemented differently with flask)

• Implement an ArticleController #110
• Implement a UserController
• Add routes logic for users and articles #108
• Connect backend to MongoDB and fill routes endpoints #111
  The controllers might embed this object
• Add authentication middleware #109
• Write pytest test suite and a corresponding github Action

[dolphinau]

In case of, I found this flag-login plugin(not tested)