Tweak admin display for authored articles

Addresses the changes asked for in this ticket: http://www.pivotaltracker.com/story/show/46394133

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.