In the unlikely, but possible, scenario where a user has no email_tokens, and has an invite record for their email address, login would fail. This commit fixes the `Invite` `user_doesnt_already_exist` validation so that it only applies to new invites, or when changing the email address.
This regressed in d8fe0f4199b5bb44fa79fa489586b4029289242c (based on `git bisect`)