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.

app/notify_client/user_api_client.py

@@ -2,12 +2,12 @@ from itertools import chain
 
 from notifications_python_client.errors import HTTPError
 
-from app.notify_client import NotifyAdminAPIClient, cache
-from app.notify_client.models import (
+from app.models.user import (
     User,
     roles,
     translate_permissions_from_admin_roles_to_db,
 )
+from app.notify_client import NotifyAdminAPIClient, cache
 
 ALLOWED_ATTRIBUTES = {
     'name',