Delete old unused FCM Push Tokens

[luke-]

We should clean up the user's push tokens after some time.
One strategy could be, for example, to send a “lastSeen” of the current push token to the database table after each (automatic) login.
We can then delete push tokens that have not been seen for more than 1-2 months.

Likewise, we could then detect push tokens that have been stored on the browser but have been deleted on the server side, and also delete them in the browser.

[luke-]

@marc-farre Maybe a task for you? But it's maybe something after bs5 not urgent at all..