Add PlatformStatsAPIClient

In API, the endpoint for the new platform admin stats page has been
moved to a platform stats blueprint. This means we now need a platform
stats client.

app/notify_client/platform_stats_api_client.py

@@ -0,0 +1,11 @@
+from app.notify_client import NotifyAdminAPIClient
+
+
+class PlatformStatsAPIClient(NotifyAdminAPIClient):
+    # Fudge assert in the super __init__ so
+    # we can set those variables later.
+    def __init__(self):
+        super().__init__("a" * 73, "b")
+
+    def get_aggregate_platform_stats(self, params_dict=None):
+        return self.get("/platform-stats", params=params_dict)