Allow non-admins to edit own registration information

The use case makes sense and I am good with this as long as existing installs are not affected. Meaning it is done with a checkbox setting on the per-host-entity Registration Settings form (a new base field), the Registration type, or global Registration settings, and an update hook sets the box to unchecked for existing sites. If you want new sites to default to checked, no problem. If putting in global settings, https://www.drupal.org/project/registration/issues/3490561 🐛 From address in registration setting is overwritten Active provides an example.

This should definitely wait until the related issue #3464736 is Fixed, so as to avoid rework.

Makes sense re existing installs.

What's tricky with regard to 📌 Replace HostEntity::isEnabledForRegistration Active the behabior we give to the new isRegistrationValidForHost() we are introducing. I'm not fond of the idea we bake into it the legacy idea that any existing registration is invalid for the host if the host is not longer available in the sense defined by isAvailableForRegistration().

But if we don't keep the legacy behavior in isRegistrationValidForHost(), then we have to deliberately implement the legacy behavior elsewhere in addition alongside calls to the new isRegistrationValidForHost().

Here's a proposal that gives us maybe the best of both worlds:

In 📌 Replace HostEntity::isEnabledForRegistration Active :
(1) isRegistrationValidForHost() only checks isAvailableForRegistration() if the registration is new or changes the number of spaces. It doesn't have the legacy behavior.
(2) The logic in RegistrationAccessHandler added in 🐛 Access & validation mismatch creating/editing a registration on a disabled host Active for 'update' is changed to consider isAvailableForRegistration(), keeping current behavior that blocks access for non-admins to edit registrations on closed hosts.
(3) The RegistrationConstraintValidator is changed to consider only isRegistrationValidForHost(). This looks like a behavior change for ordinary users, but actually its not because they can't access the registration edit form to edit it anyway if the host is closed. This will have the consequence that admins are able to edit registration information (but not spaces etc.) on closed hosts. So this is a behavior change, but probably beneficial and only for admins.

In this issue:
(4) We add a setting that give site builders control over whether RegistrationAccessHandler should or should not block access to edit depending on isAvailableForRegistration(). I suggest a global setting to keep it simple, as I think sites needs here are so various that I don't have a lot of confidence its that widely useful for more than BC.