Trigger webhooks in the backend, not the frontend

[pju-]

Currently the outgoing webhook-requests are made via XHR.

It would be better to trigger them from the backend – that way the payload-data would always be up to date.

Also the plugin could then more easily provide a "before" hook. This could make for some good workflows – e.g. of using the outgoing hook, users could trigger a build, upload to a build repository, the upload triggers a hook to deploy, then the result of the deployment is reflected in the panel.