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/user_api_client.py

@@ -226,3 +226,6 @@ class UserApiClient(NotifyAdminAPIClient):
 
     def user_belongs_to_service(self, user, service_id):
         return service_id in self.get_service_ids_for_user(user)
+
+
+user_api_client = UserApiClient()