Add logic to ensure that if someone de-activates the webhook we get notified with a critical alert

Currently, GitHub exposes an event for someone deleting the webhook (meta.deleted). But users can also de-activate the webhook an no events are fired. If we get a token with the correct permissions we can poll `https://api.github.com/orgs/electron/hooks` and ensure that we are still present in the hooks list and active === true. If not we can fire a critical alert.