Bump govuk_content_models to 8.9.0

for updates to RenderedSpecialistDocument
Expose specialist document facet labels in artefact

This is so that we can present the labels on the specialist document
show page.
Fix test: use valid slug for detailed guides

The slug validation has been tightened up a little, and the
whitehall_artefact fixture by default generates a slug like
government/some-slug.

That slug is invalid for a detailed guide, so it was causing the slug
validation to fail since the validator has been tightend up.

Adjust this test to generate valid slugs for detailed guides.
Merge pull request #172 from alphagov/expose-specialist-document-facet-labels

Expose specialist document facet labels