Change signup confirmation behaviour

[doublethink]

I do not believe this change will be a wholesale change to the expected behavior by developers. We do not advertise that unconfirmed accounts will resend the confirmation email. We only share the different behavior based on existing vs new accounts. If anything, we would need to remove one word (confirmed) from the statement "If signUp() is called for an existing confirmed user" in the documention

EDIT: TIL I think this behavior is being used by the invite function

Fixes SEC-483