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`
Right, I use factories most places, but the mail notifications have very specific names and purposes like Settings would -- you wouldn't just use a Factory to generate one, because the underlying mailer and method need to exist.

Fixtures are still appropriate there, right?