[Referee Invitations] Drop the `referee` field in favor of the `profile` field.

Currently, the referee invitations model has two fields with the same (competing) information: one for the referee Contributor field and one for the profile Profile field. Since Profile is more general, for both registered and non-registered users, it should be the definitive source of truth for the invitation. This issue involves removing this reduntant field, and hunting down all references to be replaced in the codebase.