Clear a deleted domain from the tenants that named it as their public or workspace site

Adds the domain_predelete hook that mirrors the tenant one already there.

Deleting a domain left every tenant that named it as its public or workspace site pointing at a machine name nothing answers to. That is inert while the name stays free, and stops being inert the moment an unrelated domain is created under it: the tenant then renders its notifications on that site.

No config dependency is added, deliberately: one would delete the tenant along with the domain, which is why the existing direction is a predelete hook too.

TenantDomainCleanupTest asserts both directions in one class. Two of its four cases fail against unpatched 1.x.