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`
maybe I'm missing context, but why inner classes?