PushSubscription IDL links broken

[annevk]

In https://w3c.github.io/push-api/#pushsubscription-interface endpoint and expirationTime are not links. I think that's because of them having some definitions that are used by the toJSON method, but that should probably be separated.

E.g.,

The endpoint getter steps are to return the result of getting the endpoint attribute.

To get the endpoint attribute, ...