[push-api] userVisible -> userVisibleOnly (#137)

In #131 I introduce the `userVisible` option to the `PushSubscriptionOptions` dictionary. We've since received [some feedback](https://groups.google.com/a/chromium.org/d/msg/blink-dev/_2JV9BTB1Ec/33xzm_MTzVYJ) that `userVisibleOnly` is considered to be clearer.

I'm OK with this change, and don't feel strongly in either direction. The reasoning for going with `userVisible` initially is that UAs could provide some _grace_ for pages that miss a notification or two, but are otherwise good players.

However, the intention declared by the developer says that the subscription will **only** be used for messages that result in user visible UI.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/137