Use a JSON schema for validating case studies

We're dog-fooding the use of a JSON Schema as a means to verify the structure of the JSON we are creating for pushing into the publishing API. This is still an incomplete representation of a case study and the schema will evolve as we move towards a complete representation.

Story: https://trello.com/c/OAC3a2Wx/30-incorporate-a-case-study-schema-into-whitehall
We should require content_ids to be in lowercase, to ensure that string equality works for checking equality.