I should be able to Cc: people on Notifications

Summary: This is motivated by this Backbone story. The underlying idea is Cc'ing on the resulting emails, of course, but in order to achieve it we have to introduce a notion of Notification Cc:'s.

The workaround is that we are simply going to add these people as Recipients for now. But that's sub-optimal -- we actually want a distinct concept of primary and secondary recipients.

Also, note that we currently have no notion of Recipients being able to see who else has been copied on this notification. We might want to add that.