Make the `Category.name` model attribute required

Update the model itself, and update the form tests and factories to reflect that
change.
Add tools for using Category objects in templates.

Tools all expect a Category model object and return something based on that
object. Not all tools are working. Some work, others are simply stubs that
return 'FIXME'.

Ensure doctests in `category_tools.py` are run with rest of test suite.
Update README to list dependency
Fix bad wording in item-id.html template
Make UI present more info about categories