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.
I don't really care, I'm happy to remove it, I was just providing an example of how to do system mail notifications.

And generally, I agree for any other situation other than this one.  What's the argument against doing it?  I did some research and found opinions both ways. 

The option is off by default, so it's opt-in. If an admin is creating a bunch of accounts for new users, it makes sense to be able to send a welcome mail like this to tell the user their new credentials, and a link to login -- they'll have to do this manually by hand, and it would probably be by mail or an internal chat system anyway.

I don't see this option as being really any different than say, sending a link with a token to login via e-mail.