Split models to prevent circular imports

This commit is the first step to disentangling the models from the API
clients. With the models in the same folder as the API clients it makes
it hard to import the API clients within the model without getting a
circular import.

After this commit the user API clients still has this problem, but at
least the service API client doesn’t.

tests/app/notify_client/test_notify_admin_api_client.py

@@ -4,8 +4,8 @@ from unittest.mock import patch
 import pytest
 import werkzeug
 
+from app.models.service import Service
 from app.notify_client import NotifyAdminAPIClient
-from app.notify_client.models import Service
 from tests import service_json
 from tests.conftest import api_user_active, platform_admin_user, set_config