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/inbound_number_client.py

@@ -14,3 +14,6 @@ class InboundNumberClient(NotifyAdminAPIClient):
 
     def get_inbound_sms_number_for_service(self, service_id):
         return self.get('/inbound-number/service/{}'.format(service_id))
+
+
+inbound_number_client = InboundNumberClient()