Tweak admin display for authored articles

This fills the admin screens with (yet more) conditionals - the authored
article really should be its own edition type. When we come to improve
the way that the images work for authored articles, splitting it out
into its own edition type could well make sense.

Tested via end-to-end cucumber feature as only significant changes are
admin helper code.
Merge pull request #461 from alphagov/authored-article-admin-display-changes

Tweak admin display for authored articles

http://www.pivotaltracker.com/story/show/46394133