Support blacklisting of webhook servers

[eminano]

While it's the subscription server exposes an endpoint to unsubscribe from the webhook notifications, it relies on the receiver to make a request. It can happen that subscribed servers are in a bad state (always returning error status), or down altogether. Currently all the subscriptions are kept regardless, which could be exploited.

One solution could be to force the subscriber to re-subscribe after a set time period, but this can be annoying, since we force the receiver to constantly re-subscribe, leading to missing notifications. Another solution could be to blacklist servers that are not healthy, keeping them for a period of time before deleting them altogether.