Fix artefact registration

This fixes a couple of things with panopticon artefact registration:

* If creating/updating an artefact fails, we now return a 400 status with error messages (previously we were still returning 200/201 with no indication that anything had failed.)
* Registration didn't support travel-advice slugs (of the form `travel-advice/<country_slug>`).  This required an update to govuk_content_models.

I've also tidied up the setup of webmock and database_cleaner.