MVP presenter for case studies

Just to get a failing test passing and learn something. Learned that generating
HTML from govspeak outside of a view context is going to be a whole heap of
fun...
Render the govspeak body as HTML
Rename test file to match thing it tests
Wire in case study representation

Case studies use their own representation when pushing to the publishing API so
we can use them as a test case through the new stack.
Refactor PublishingApiPresenter::CaseStudy

Remove the duplication between the presenters with inheritance
Tidy up use of LeadImagePresenterHelper

Removes duplication and make public/private interface clearer
Publishing API: Include an image for case studies

If the case study or it's org has an image available, we include it in the JSON
sent to the Publishing API.

Note that this code will not fall back to the "placeholder" image if neither the
case study or the organisation doesn't have an image. This is intentional as the
responsibility for deciding what to display if no image is available should lie
with the public-facing application that renders the content.
Rename first published key to match method name
Data migration to push case studies to content store
Merge pull request #1838 from alphagov/case-studies-to-publishing-api

Case studies to publishing api