Allow artefact registration to support whitehall slugs

This needs to support slugs with consecutive '-'s in them, and
aditionally some with a .locale suffix.

The former is addressed by the content_models bump, whereas the latter
additionally needed support in the routes.

Fixes [[#67540714](https://www.pivotaltracker.com/story/show/67540714), [#67541692](https://www.pivotaltracker.com/story/show/67541692)]