Feature PT: 78649336 - web interface for mailer

This PR  https://www.pivotaltracker.com/n/projects/982890/stories/78649336 should satisfy the need for a web based possibility to create a newsletter to be sent out to all subscriber.

it can be configured via Settings as either beiing instantly sent out - or  for the status quo of sendGrid to be sent out in chunks so the quota doesn't get exceeded. 

Besides that in introduces the possibility to restrict specific actions to a given set of defineable users.

Since this method is used only in the `NewslettersController`, can this method be placed there?