Enable to skip sending reconfirmation email when skip_confirmation_notification! is invoked

We could always generate a confirmation token but not sending a
confirmation email by invoking the skip_confirmation_notification!
method when creating the account.
But there were no way to do that when we were turning on reconfirmable
and updating email.
Merge pull request #2475 from tkhr/enable_skipping_reconfirmation_notification_but_regenerate_confirmation_token_2

Enable to skip sending reconfirmation email when reconfirmable is on and skip_confirmation_notification! is invoked