Add organisations to case studies content-store json.

Add a links hash in the case_study presenter, for now just containing content_ids for lead and supporting organisations.
Good spot, corrected. I reckon it might be time to think about pulling out these validations to some common thing that all the apps can use. We're already duplicating the code in [content_store](https://github.com/alphagov/content-store/blob/master/app/validators/uuid_validator.rb) and [hmrc_contacts](https://github.com/alphagov/hmrc-contacts/blob/master/app/validators/uuid_validator.rb).