Add our new Stripe webhook to the payment dashboard

This adds the new Stripe webhook endpoint to the "payment" Grafana dashboard.

Also keep the "charge" API so we see if someone calls that (?).

Keep the "redemption" dash the same.

(Successfull webhook calls) * 6.50 USD should give us the money we made.
The redemptions dash also includes redeemed (gratis) vouchers (of which we give 3000 out to notify me). (Right?)