Refactor: Improve Pylint score for apps/challenges/models.py to 9.54

[Golixco]

Fixes #4557

Summary:
Refactored apps/challenges/models.py to address Pylint errors and warnings, significantly improving code quality and fixing potential runtime bugs.

Key Changes:

• Bug Fixes (Crash Prevention): Updated __str__ methods to explicitly return strings (str(self.field)), preventing potential TypeError crashes when fields are None (Fixes E0307).
• Modernization: Updated legacy super(Class, self) calls to Python 3 style super() (Fixes R1725).
• Optimization: Refactored comparison logic to use the min() builtin for cleaner, more readable code (Fixes R1730).
• Cleanup: Fixed indentation errors, trailing whitespace, and redundant string formatting.

Results:

• Pylint score increased from baseline to 9.54/10.

[Golixco]

It seems the Travis CI build is failing during the Docker build stage due to a scikit-learn hash mismatch in worker_py3_8.txt. My local code quality checks (Pylint) are passing at 9.80/10. Once the repo's dependency issue is resolved, this PR should turn green, Please I request the maintainers to have a look here. Thank You!