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/platform_stats_api_client.py

@@ -9,3 +9,6 @@ class PlatformStatsAPIClient(NotifyAdminAPIClient):
 
     def get_aggregate_platform_stats(self, params_dict=None):
         return self.get("/platform-stats", params=params_dict)
+
+
+platform_stats_api_client = PlatformStatsAPIClient()