New category navigation

This is a bit useless until the new IA is fully released as there is an assumption that articles are under 2 levels of categories, not the 3 we currently have.

The nav list is in a partial that's just plonked into the article body so some front-end attention is required.

Potentially contentious is the introduction of a fake article repository which is being used to get around the transitional state of the IA.