Delete push notification registration on logout and account deletion (client-side)

[mplorentz]

Currently after logging out or deleting your account you continue to receive push notifications for the account you were signed in as.

To fix this we need to tell our push notification server to delete the association between the user's pubkey and their APNS token when the user logs out or deletes their account.

Currently there is no API for this. This will need to be coordinated with planetary-social/nos-notification-service-go#70.