Automatically renew the push notifications subscription

Continuing the discussion from Android push notifications stop working:

Looks like Chrome will finally implement the pushsubscriptionchange event which is already supported by all other browsers who implement the modern web specs.

We have plenty of cases where the notifications cease to work all of sudden, and this may be one of the reasons why.

Implementing this means handling the event at the service works, plus handling the subscription renewal with minimal (0 if possible) effort from the user. It may be needed to save a few bits of information during subscriptions to make it possible, so you know which subscription you gotta replace.