Move display_type logic into model

All editions have a display_type which we use to display what type of
thing an edition is. Moved the publication and consultation logic from
helpers into their respective display_types.
Add display_type to all atom feed titles

Also refactored the tests so that the format is only defined in one
place. This should make it easier to make changes to the format of atom
feeds.
Remove temporary variable for clarity
Merge pull request #156 from alphagov/add-type-to-atom

Add type to atom