Make the admin navigation only contain 3 items

The admin nav bar was getting very very long. It was decided that
everything other than "Documents", "Document series" and "[My
department]" should be moved under a sub-nav item.

https://www.pivotaltracker.com/story/show/44907221
https://www.pivotaltracker.com/story/show/45162979
Merge pull request #280 from alphagov/admin-nav

Make the admin navigation only contain 3 items