Allow the "Confirmation Email" in "Notification Settings" to be multi-choice

[royberris]

Is your feature request related to a problem? Please describe.
We would like to sent multiple e-mails when an order finalized. Right now this is not possible. I can only send the same e-mail to multiple BCC's.

Describe the solution you'd like
When selecting an e-mail, you'd allow it to be multiple choice. Where 1 or more e-mails are sent.

Describe alternatives you've considered
I could hook in to the OrderFinalizedNotification just like it is done with the confirmation e-mail. But it would be better to be able to configure it at one place. When doing it manually I can't configure which e-mail(s) are sent through the back office.

Additional context

You might also want to consider it for the error e-mail. But I think this is less needed.