Can see/add/remove categories for resources
Start to re-work Book nav

At the moment, most of the links in the Books dropdown are broken.  When
books all have categories assigned, we can get them working again.

The trick will be to replace them with
  link_to category.name, category_resources_path(category)
Use link helper, to prevent xss.
Stop routing to URLs that don't work

@precedex If you need to keep these URLs (e.g. if you've published them
elsewhere) we can get them working again after you create the
categories.