[w3c/push-api] Allow atomic push resubscribe (#292)

I was wondering if we can consider having an atomic resubscribe method for push? Apparently there are sites that unsubscribe and then try to subscribe again and then they lose their subscription if the browser is closed, for instance. It would be ideal if we were able to subscribe another channel and then if it succeeds, we remove the old channel.

After chatting with @beverloo, it sounded like we should have something like `PushManager.refreshSubscription()` that invokes the [refresh algorithm](https://www.w3.org/TR/push-api/#subscription-refreshes) and would return a `Promise<void>` that'd be resolved when the `pushsubscriptionchange` event has finished executing.

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