This is a simple proxy between HelloSign's Webhooks and Zapier.
Any requests POSTed to the API will be forwarded to the relevant Zapier URL.
The request path will be appended to https://hooks.zapier.com/hooks/catch/,
which is the format for Zapier Webhooks. For example, a POST to /foo/bar
will proxy to https://hooks.zapier.com/hooks/catch/foo/bar.
In order to ensure that POST requests are coming from HelloSign, the API will check for the presence of a User Agent and Body Hash. Bad requests will result in a 401 response.