Add ability to reorder items and categories

- explicity define item many-to-many with list/category
- pass intersection models to add/remove item/category
  - removes need for hidden input fields
  - remove unneeded forms

- replace boilerplate in view with get_object_or_404

- items and categories assigned display order on association
- items and categories can increment/decrement display order
- items and categories and be explicity assigned display order

- items and categories are displayed in template via sortedList function

- add jQuery UI
- upgrade to foundicons 3