Add credentials for slack-notifications on PP prod and PP staging

[anna-parker]

Add slack tokens as described in loculus-project/loculus#2396.

1. Create a new channel: #pathoplexus-notifications and slack bot for pathoplexus (give permissions: files:write, incoming-webhook, chat:write, files:read), invite to channel.
2. Seal secrets:

kubectl create secret generic slack-notifications --from-literal=slack_hook=KEY --dry-run=client -o yaml > secret.yaml
kubeseal --format yaml  --controller-namespace=kube-system --controller-name=NAME --scope=cluster-wide < secret.yaml