Return top-priority category as parent for top-level page.

This should resolve gma/nesta#147.
I'd forgotten/hadn't noticed that we currently sort categories alphabetically. That'll only be for presentation purposes in 

I think I'd rather we went for two methods here, one called `#categories` that just returns the pages in the order that they're mentioned in the metadata, and another (`#sorted_categories`) that sorts them alphabetically.