Inherit don’t duplicate API client constructor

This removes some code which is duplicative and obscure (ie it’s not
very clear why we do `"a" * 73` even though there is a Very Good Reason
for doing so).

app/notify_client/__init__.py

@@ -16,6 +16,9 @@ class NotifyAdminAPIClient(BaseAPIClient):
 
     redis_client = RedisClient()
 
+    def __init__(self):
+        super().__init__("a" * 73, "b")
+
     def init_app(self, app):
         self.base_url = app.config['API_HOST_NAME']
         self.service_id = app.config['ADMIN_CLIENT_USER_NAME']