Use 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 content store. This is still an
incomplete representation of a case study and the schema will evolve as we move
towards a complete representation.
content_id must be lowercase
Allow image caption to be null

There is a potential discussion to be had about whether we should exclude keys
when the value is nil on optional fields, but that's for another time.
Merge pull request #1852 from alphagov/case-study-json-schema

Use a JSON schema for validating case studies