Deleting a domain breaks referential integrity

When we delete a domain, we don't check if there are resources that are owned by it. Depending on the database this will either leave the DB in a somewhat inconsistent state...or fail with an integrity check error.

A wider question is what SHOULD happen if someone tries to delete a domain? Logically, we should delete all projects, users and groups that are owned by it - although it would be unfortunate if someone did that in error! I suggest we should ONLY allow a domain to be deleted if it is first disabled, as way of preventing inadvertent deletion - and then we should really delete all the entities owned by that domain.