Notify for 500's on state_change
Notify for 500's on state_change

By only alerting every 60 minutes we would miss some 500s if they
occurred within an hour or the original alert.

By changing to `state_change` we should see alerts for all 500s.

alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws