Handle unpublishing format

This renders a bare-bones unpublishing, with no styling.

![screen shot 2014-12-10 at 16 06 26](https://cloud.githubusercontent.com/assets/3687/5379087/81da7fb4-8086-11e4-9f73-d9c019b160ec.png)

The build will fail until https://github.com/alphagov/govuk-content-schemas/pull/8 is merged.

The generic controller is maintained, but now we have format-specific presenters and view templates. Whilst we only support two formats, putting this logic inline in the controller feels OK. Once we are handling more formats though, this logic will want to be extracted elsewhere. We'll also likely want to organise the view templates into folders at some point so that related partials can be kept together.

Story: https://trello.com/c/o1EFKTaj/60-government-frontend-handles-unpublishing-format-content-items