We need to cope properly with invitee trying to sign up for Querki a second time

(Bug, To be Fixed , Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Invites, Security
Summary: At the moment, if you receive an invite to a Space, sign up for Querki, then later click on the invite and sign up again, I believe we will crash (correctly) in createUser(). We should detect and handle this earlier.
I believe the correct response is, in handleInvite(), we should check the given Identity; if it's not a SimpleEmail Identity, then instead of offering the signup form we should just show them the login form. We should probably prompt them that they have already signed up for Querki, and we should offer the password-reset dance.