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.
Merge pull request #391 from alphagov/restrict_leaderboard_to_gds_editors

Really restrict viewing of the leaderboard to only GDS Editors