Improve structure & handling of custom taxonomy URLs

Matt Boynes (@senyob) helped me change things around so that custom
taxonomy URL slugs can be pre-pended with art, grouping all art
content together under the same URL structure. Many thanks to him
for the assistance. Beer is owed as thanks.

Additionally, I

* cleaned up the labeling for our custom post types and taxonomies
* installed the Debug Bar & fixed an undefined args issue it caught