Add main edition type to admin edition list

Also added subtype to edition show pages because it
uses the edition_type helper.

https://www.pivotaltracker.com/story/show/57977482
Merge pull request #912 from alphagov/57977482-add-main-type-to-admin-edition-index

Add main edition type to admin edition list

https://www.pivotaltracker.com/story/show/57977482