Display notice about archived documents

- Update presenter to know if an item is archived. We know it is
  archived due to the presence of an archive notice in the details hash
- Create page_title method to automatically prepend 'Archived'. In the
  future this would use an I18n method to get a localised archive title.
  However, in the current version of this code none of the archive
  notice can be translated so leaving that as a future exercise.
Downcase format type to match sentence case
Merge pull request #10 from alphagov/archived

Display notice about archived documents
WIP tests for ContentItemPresenter