Initialise clients outside the app

This avoids the annoying problem where you can’t import a client unless
the app has already been initialised.

app/notify_client/notification_api_client.py

@@ -92,3 +92,6 @@ class NotificationApiClient(NotifyAdminAPIClient):
         )
 
         return self.get(url=get_url)
+
+
+notification_api_client = NotificationApiClient()