webhook-manager: Allow disabling webhooks

[chrismytton]

There should be a way to disable webhooks for a certain application, this might be done manually at first for ones that consistently fail. In the future we might want to automate this so that if a webhook consistently fails n times of for n days in a row then we can automatically disable it and send the owner a message or something.

[chrismytton]

If we did https://github.com/everypolitician/everypolitician/issues/307 then just unchecking every event would effectively disable webhooks, so this then wouldn't be needed.