List webhook subscriptions

Webhook subscriptions deliver events such as video.created, tutorial.created and project.updated to your server as signed POST requests (X-Tutorializer-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.