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`
@ohadlevy Ok, updated everything except for this one.

Like I mentioned on IRC, the main concern is the user's roles aren't saved yet, so it would probably need some javascript to update the mail notification page once roles are selected, and my JS  skills are pretty limited, so this would take me a while.