[push-api] Align "push client", "push server", and "push service" with protocol (#113)

I think we can simplify the spec terminology and further align with the protocol [1] as follows:

* **push client** is only used in the diagram, and whether it is part of the user agent or a separate entity running on the same system is an implementation detail. The spec otherwise never refers to this term. It can be removed from the diagram.
* **push server** overlaps an awful lot with **push service** and one could simply state once that a **push service** has a server. After that, I think we can avoid using this term and always refer to the **push service** instead. The protocol spec already largely avoids this term, and I think we could remove it entirely.
* **push service** the definition of this one can probably be clarified, and take a piece or two from the term **push server** which I propose can then be removed.

1 - https://martinthomson.github.io/drafts/draft-thomson-webpush-http2.html

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/113