Allow JS transformations in Outbound hooks

Simple as that - it would be great to have more control over how the outbound webhook payload looks like, there are various consumers which may not always be too friendly to form data (like [Webhook](https://github.com/adnanh/webhook/) or Jenkins).

Start by locating the outbound hook implementation and tracing how its webhook payload is assembled. Define the supported JavaScript transformation behavior and verify that transformed payloads can be consumed by form-data-unfriendly integrations such as Webhook or Jenkins.

Written by the indexing model from the issue text.