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_activity.py

@@ -9,7 +9,7 @@ from flask import url_for
 from freezegun import freeze_time
 
 from app.main.views.jobs import get_status_filters, get_time_left
-from app.notify_client.models import Service
+from app.models.service import Service
 from tests.conftest import (
     SERVICE_ONE_ID,
     active_caseworking_user,