Rename references to CmaCase to Document

In the next commits we will be refactoring this code to work with both AAIB reports and CMA cases so lets not be explicit about things being a "case"
Fetch the schema based on the finder slug

Instead of hard coding the schema type use the slug from the request parameters to decide which schema to use.
Merge pull request #28 from alphagov/feature/add-aaib-reports

Allow handling of AAIB Reports