Fix cyclic import dependency in challenges/aws_utils.py

### Description

Currently, we can't import `Challenge` model or any method from `challenges/utils.py` in [challenges/aws_utils.py](https://github.com/Cloud-CV/EvalAI/blob/master/apps/challenges/aws_utils.py) because `aws_utils.py` is being imported in `challenges/model.py`. If we try to import any util methods or models from `challenges/utils.py` or `challenges/models.py` in `challenges/aws_utils.py` it fails because of cyclic imports.

### Fix

Refactor `challenges/models.py` to fix cyclic imports