Move edition duplication out to its own class

There's enough logic here that it's better living in its own class rather
than buried in a controller action.

This also revealed some improvements we could make to the structure of
assignment of editions to users elsewhere in the code so I cleaned that
up a little.
Move duplication tests out to unit test

Our functional tests (controller tests) don't need to worry about
the complexity of duplication. That can now be covered in a unit
test of the duplicator class.
Update description of the new_format parameter
Merge branch 'master' into extract-edition-duplication

Conflicts:
	app/controllers/admin/editions_controller.rb
Merge pull request #85 from alphagov/extract-edition-duplication

Extract edition duplication