Make all config configurable by envs

Merge settings.py and config.py files. Make all values configurable by env values, even thou we don't need to set them in infra, we might need later.

Eg,

class Config:
    DEBUG = ast.literal_eval(os.getenv("DEBUG", "False"))
    # Auth
    CERN_OIDC_CLIENT_ID = os.environ.get("CERN_OIDC_CLIENT_ID")
    ...