Add travel advice

This adds travel-advice content to the content_api.  These requests will be for artefacts with a slug of `travel-advice/<country_slug>`.  

The logic for these is a bit different:
* The requests should only 404 if an invalid country is requested.
* With a valid country, but no live artfact, a mostly blank response is returned that includes the country details.

The Country model is temporarily driven from a YAML file.  This will be replaced by something that consumes an API from Inside Gov when this API has been built.