Move `PushNotificationSender` into separate app

[timobrembeck]

Motivation

We outsourced all clients of external APIs (e.g. DeepL, Nominatim, SUMM.AI, GVZ) into separate Django apps.

Proposed Solution

For consistency reasons, I suggest to move the Firebase API client to a separate app as well.

Alternatives

Just keep it the way it is

Additional Context