Render full menu tree in Drupal 8 version

Got this working with http://drupalcode.org/project/gratis.git/commit/8e9ac77

Inspiration from:
http://drupalcode.org/project/bootstrap.git/blob/refs/heads/8.x-2.x:/boo...

Now we just need to theme it like the Drupal 7 version.

Opening back up, it's broken possibly because of #2207893: Convert menu tree building to a service.. Also see this: Drupal 8 Menu Tree issue in my contrib theme possibly because of “Convert menu tree building to a service.”

Fixed again.