* if the service issuing the invite does not have permission to edit
auth types, don't let them do anything. This will stop them turning
existing email_auth users back to sms auth
* if the user hasn't got a mobile number, but the invite is for sms
login, don't do anything either. They won't have a phone number if
they signed up via an email_auth invite previously.
in these cases, we accept the invite and add the user to the service
as normal, however, just don't update the user's auth type.