feat: Notification system

[joelbarmettlerUZH]

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What does your PR change?

I added a new notification system that notifies user on system events. These events can be triggered by admins in the frontend or by using the backend service. @davwys can we have a talk about the possibility of introducing i18n in the backend?

[sonarcloud[bot]]

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3.3% Coverage
2.3% Duplication