Refactor registration_waitlist RegistrationValidationEventSubscriber

Studying this I've arrived at a conclusion:
(1) The current subscriber may be significantly broken, as shown by the failing test I've added.
(2) If we fix hasRoom() so it obeys its interface and returns a truthful answer instead of always returning true, then all the subscriber needs to do is to enhance the message/code if the waitlist is full.
(3) If the subscribers function is simple like this, then it makes sense as a subscriber and I don't see a particular need for 📌 Waitlist validation subscriber better as swapped plugin class Active .

Unexpected test fail, needs more work.

john.oltman → made their first commit to this issue’s fork.

This is a nice improvement, thanks @jonathanshaw.

There is a failure in phpcs that can be ignored re: use statements in the Drush command, it is due to a regression upstream in the coder contrib module, and the maintainers there are working to fix it.

For now, committing only to 3.3.x, pending discussion in 🌱 Roadmap for 3.4 release Active around the future of the 3.1.x branch.

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