Allow hiding RSS-to-email emails from the archives

[benborgers]

An option when setting up an RSS-to-email automation that hides it from archives, the way that you can when sending an email.

https://secure.helpscout.net/conversation/2737830559/5920

[rkingett]

This would also be good for newsletters that mirror things from private places, like a private blog, etc, and it would also be good for creating premium content too!