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)

• [ ] #110
• [ ] Implement a UserController
• [x] #108
• [ ] #111 The controllers might embed this object
• [ ] #109
• [ ] Write pytest test suite and a corresponding github Action

[atxr]

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