Fix error when attempting to create without a slug

This was attempting to register the blank slug with url-arbiter, which
returned a validation error.  This avoids calling url-arbiter for these
requests because we know they'll be invalid.
Merge pull request #225 from alphagov/fix_blank_slug_error

Fix error when attempting to create without a slug
Bump govuk_content_models to 22.1.2

This version bump adds the new artefact type of MAIB Reports being
published by specialist_publisher
Merge pull request #226 from alphagov/bump-govuk-content-models-to-22-1-2

Bump govuk_content_models to 22.1.2
Bump govuk_content_models to 22.2.0

This version bump includes the RAIB Report artefact kind and a breaking
change that removes safe_html validation in non-govspeak fields. This
should be fine, as Rails takes care of these fields anyways.
Merge pull request #228 from alphagov/bump-govuk-content-models-to-22-2-0

Bump govuk_content_models to 22.2.0