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/template_statistics_api_client.py

@@ -26,3 +26,6 @@ class TemplateStatisticsApiClient(NotifyAdminAPIClient):
         return self.get(
             url='/service/{}/template-statistics/{}'.format(service_id, template_id)
         )['data']
+
+
+template_statistics_client = TemplateStatisticsApiClient()