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/complaint_api_client.py

@@ -13,3 +13,6 @@ class ComplaintApiClient(NotifyAdminAPIClient):
 
     def get_complaint_count(self, params_dict=None):
         return self.get('/complaint/count-by-date-range', params=params_dict)
+
+
+complaint_api_client = ComplaintApiClient()