[Bug] APNS tokens are stored for all past pubkeys of the app

[dcadenas]

We need to allow Nos to delete push notification registrations from our notification service to fix planetary-social/nos#1613

The APNS token identifies the app and the device. Each time we logout and use the app with different pubkeys they are all registered as using the same APNS token. This means that the current pubkey logged in to the app will receive notifications for all the previous users that used the app.

Implementation

Probably we should reuse the existing registration event and add a new property to the JSON to specify whether we are registering a pubkey or deregistering it. And when we receive a registration event with a apnsToken that matches an existing pubkey->apnsToken pair in our database then we delete the old pair and replace it with the new one.

Consider bundling this work with planetary-social/nos#852.

The deployment of this will need to be coordinated with planetary-social/nos#1613.