Add a Registration::isNewToHost() method

There's a fair number of places where it's not easy to say whether we should use isNew() or isNewToHost(), but I reckon I've got most of them. And missing one won't cause any problems for existing sites, it might only lead to a difficulty in some obscure edge case when using registration_change_host.

Looks really close - requested a few minor changes.

Working on it.

Made the changes as requested. Please review.

Automatically closed - issue fixed for 2 weeks with no activity.