DomainName states 'external' and 'special' should be attributes

[ph448]

This is prompted by the fact that the current implementation loses the 'external' detail if the DomainName gets set to 'deleted' then correctly vaildates again.

DomainName.status is the scope of availability of the hostname for use on a virtual host, whereas 'special' and 'external' states are orthogonal to this: a name is 'external' when it is not a domain under .cam.ac.uk and it is 'special' if the user sets it to special, usually meaning some quirk that makes it impossible for us to later rescind the name if required.