Case studies to publishing api

This updates whitehall to push partial content for published case studies to the Publishing API so we can build an MVP frontend application. Things to note:

* The format is no longer `"placeholder"` and is now `"case_study"`, meaning routes will be registered for case studies
* The frontapp has not changed so the existing `"whitehall-frontend"` application will continue to render case studies on GOV.UK
* This is pushing a partial representation of case studies (body, first_public_at and image information); document history and "links" (e.g. the meta data) are not included yet.
* There is a data migration to push all case studies into the content store.
* An image is only included if the case study has one or it's lead organisation - unlike the presenter code in whitehall, the publishing API representation does not fall back to the placeholder image, instead it's left up to the frontend app to decide what it does when an image isn't present.

As part of this work, an object was pulled out to delegate rendering of govspeak to HTML outside of the view context. I want to take this to its logical conclusion and replace the current set of helpers with a single object that handles govspeak rendering, but that's a fairly chunky piece of work that can come in a separate pull request.

Story: https://trello.com/c/dDayyF5R/19-publish-a-single-edition-format-to-the-content-store. Part of the work to build an [MVP frontend application](https://gov-uk.atlassian.net/wiki/display/WH/New+Whitehall+frontend+tracer+bullet).