Code cleanup

[Gonzom]

Issue includes:

• [ ] Fix Mypy errors
• [ ] Fix grammar errors
• [ ] Add TODOs
• [ ] Set a consistent naming style in CSS files
• [ ] Consolidate identical functions into one. Create a fake user is an example of function which is created several times in different places.
• [ ] Use Fastapi.status instead of plain numbers
• [ ] Other miscellaneous fixes

[Gonzom]

Progress: #250, #258, #291