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/test_manage_users.py

@@ -5,7 +5,7 @@ from bs4 import BeautifulSoup
 from flask import url_for
 
 import app
-from app.notify_client.models import InvitedUser
+from app.models.user import InvitedUser
 from app.utils import is_gov_user
 from tests.conftest import (
     SERVICE_ONE_ID,