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 seed their own mail notifications, see
db/seeds.d/16-mail-notifications.rb for an example.
Should ```set_url``` and ```load_metrics``` be private?