Enable to skip sending reconfirmation email when reconfirmable is on and 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.
By merging this PR, it will be possible :)

I also made it unconfirm the account when the email has been changed in commit 76ab508.
If there's any particular reason why it should stay confirmed even though the email has been changed and reconfirmable is on, tell me and I'll delete that commit :)

Thank you.