List webhook subscriptions

Webhook subscriptions deliver article.created, article.updated, article.deleted, blog.created and contact.created events to your server as signed POST requests (X-Polyblog-Signature: t=<timestamp>,v1=<hex HMAC-SHA256 of "timestamp.body">). An endpoint failing 20 times in a row is disabled automatically. Subscriptions are managed with an operator access token; the secret is only returned once, on create.