Tidy up some particularly complex bits

Based on looking through reports in https://codeclimate.com/github/alphagov/publisher I picked out a few areas that could do with simplification and made a few changes:

* Extracted an EditionProgressor class to handle moving editions through states
* Extracted an EditionDuplicator class to pull together logic for creating new editions
* Began to break RootController#index into smaller methods as a basis for further refactoring
* Tidied up some language for consistency and reorganised a few helpers along the way