[push-api] Rename (un)register to (un)subscribe (#98)

@costinm mentioned that it would make more sense to use the term (un)subscribe instead of (un)register, since the app is not registering for an account, instead it's subscribing to a source of messages (this also matches better with terminology like Pub/Sub).

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https://github.com/w3c/push-api/issues/98