Enable alerting

This adds a Slack alerter to Grafana - for staging and production, but not for local dev.

To make it work, it needs a new secret in private-keys/grafanaEnvironment looking like the template in morph/grid/local/private-keys/grafanaEnvironment and the secret API endpoint URL I saved in this 1Password entry.

Caveat: On deploy, this will emit a warning that we can safely ignore: trace: warning: Notifier secure settings will be stored as plaintext in the Nix store!

Fixes privatestorageops#392.