Really restrict viewing of the leaderboard to only GDS Editors

- Currently, anyone with any permissions guessing the `/leaderboard`
  route can view the leaderboard as the GDS editor permission check only
  hides the menu option. Hide the page as a whole for those people not
  GDS Editors.
I don't think it makes any practical difference. We display `notice` and `alert` the same way - as a Bootstrap `warning`: http://getbootstrap.com/components/#alerts.