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`
Yea, scopes work fine, e.g. something like `Host::Managed.authorized_as(user).with_os` works fine.  Haven't seen anything odd with this, been running it for a few weeks