Add Slack Webhook for new GHI and Daily CI

~~We are currently blocked from doing this b/c neither @kessplas nor @texastony have access to the webhook URL, but once that is addressed, we should make it so the Daily CI and updates to GHI triggers a slack notification.~~

The wonderful @rishav-karanjit has unblocked us!
#162 adds a webhook for Daily CI.

However, we should not add the Issue Notification wehbook until launch, as we do not want to spam the team channel with kessplass/texastony notifications.

Prüfe zuerst Issue #162, da es den Daily CI-Webhook abdeckt und möglicherweise den relevanten Integrationspunkt zeigt. Die verbleibende Arbeit in diesem Issue ist der GHI-Issue-Notification-Webhook, der erst nach dem Launch hinzugefügt werden sollte und Spam in Teamkanälen vor dem Launch vermeiden muss.

Vom Indexierungsmodell aus dem Issue-Text verfasst.