Add AAIB & CMA to app ownership const

Add aaib-report and cma-case to the FORMATS_BY_DEFAULT_OWNING_APP constant. This will allow us to key of this in apps which consume the api rather than surfacing the document_type for all artefacts
Validate slug for all SpecialistDocument types

..not just the one with a format of specialist-document. Since we have now introduced more types of document lets pull the document types from the FORMATS_BY_DEFAULT_OWNING_APP mappings and remove the ones we don't need.
Merge pull request #185 from alphagov/feature/update-accepted-document-formats

Update accepted artefact formats