Subscription should still be possible later even if denied at some point? #183
Description
https://www.w3.org/TR/websub/#subscription-validation says:
The subscription MAY be denied by the hub at any point (even if it was previously accepted). The Subscriber SHOULD then consider that the subscription is not possible anymore.
Maybe instead of "subscription is not possible anymore" it could probably be weakened to "subscription has been terminated/invalidated" -- the denial of the subscription may be without prejudice, i.e. the subscriber may be allowed to resubscribe.