misc fixes, and new json schema

Initial json schema. For now only the first example in the specification works.
Can we use [this regex](https://github.com/alphagov/hmrc-manuals-api/blob/master/lib/valid_slug/pattern.rb#L6) for slugs? It doesn't allow dashes at the start or end of the string, and doesn't allow multiple consecutive dashes. It does allow numbers, though - we probably don't need to use numbers here, but is there any reason to exclude them?