Use decorator instead of django settings

[efagerberg]

Instead of doing

DATABASE_ROUTERS = ['django_model_deprecater.routers.DeprecatedModelRouter']
DEPRECATED_MODEL_ROUTER = {
    # Whether to allow migrate if model deprecated
    'check_allow_migrate': False,
    # Whether to allow relations if a model in the auth app is involved
    'check_allow_relation': False,
    'models': {},
}

It might be better to handle the model collection via a decorator, then once collected those models can be handed to the router.

To preserve the per environment capability of using settings, there should be some environment to warning or exception mapping.

Using a decorator might also serve as an entrypoint to detect pure python usages of the model and raise the warning or error as well.