I should *not* be able to use email address format for a Display Name

(User Story, Investigate , Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Invites, Users and Identity
Summary: If I can have an email address for my Display Name, that makes it too easy to impersonate somebody.
This will become even more critical once we unify the invitation pathways: it leaves room for somebody to intercept somebody else's invitations.
I don't think we need to be terribly clever about this -- just make it illegal to use the standard x@y.z format, and provide an error message of, "You can't use an email address as your Name."