Webhooks: Email notification upon failed retries

[birkjernstrom]

Currently, our webhooks support retries (max 5) out of the box and with 10s delay * number of retries, e.g 10s after 1st, 20s after 2nd, 30s after 3rd etc.

We received the feature request that in case all 5 retries fail (customers server is down) they would want an email notification. So that they're certain they will never lose a given webhook (cancelled subscriptions to turn off certain offerings).