fixes #7586 - user preferences for receiving mail notifications

Adds a framework for user-selectable mail notifications.  The work is
still done in ActionMailer classes and launch by rake in cron, however a
wrapper called MailNotification is used to provide RBAC and make the
association with users.

Plugins may add their own mail notifications like they would add settings, see `mail_notification/foreman.rb`
It's only used below to provide the same interface to per-user localized group mail as the normal ActionMailer single mail object.

Would you put it somewhere else/do it a different way? In Java, I'd do it as an inner class.