Add a register permissions validation constraint

I don't love the way this does access-like logic in a validation constraint and makes validation depend on the current user, but I think it's right.

I'm slightly surprised that adding it doesn't mean we can't take away code from elsewhere, but I can't see where.

Looks really good, asked for a few tweaks

(1) I've implememented AllowedRegistrant as you requested.

I actually prefer
RegistrantUnique
RegistrantAllowed

or even better
RegistrationHasUniqueRegistrant
RegistrationHasAllowedRegistrant

To keep the constraints grouped by their rough type.

(2) There's a certain duplication of logic between RegistrationManager::getRegistratntOptions() and the 2 contraints. But I don't see an easy way to mitigate that; DRY might be overengineering here.

Added commits, I fixed a couple of minor errors. Then I saw that removing the options check (you did at my request) broke some tests that look for that condition explicitly. And now I am second guessing myself on that, so I put the options check back in and will eventually create a separate issue to ponder whether that should be removed or not.

There are some other test errors that should mostly be solvable by giving the test user the appropriate create permissions so they can get past your new validator. I will leave that to you for now, if you don't get to it today I should be able to handle tonight.

The coder module update has not materialized as soon as hoped, so I addressed it here. This means we can look at the build pipelines in Drupal CI and know that all stages should pass. It was getting tiresome to have to check the validate stage to see if the failures were just the two related to upstream, or additional ones that needed to be corrected.

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