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/main/views/accounts/test_show_accounts_or_dashboard.py

@@ -1,7 +1,7 @@
 import pytest
 from flask import url_for
 
-from app.notify_client.models import User
+from app.models.user import User
 from tests import user_json