Fix bug with `_items` template tag.

Ensure that function cannot return duplicate items.
Make category UI more cohesive

Update "breadcrumb" navigation links for all /category/* pages by prepending a
link to the /item/ page. This makes it more clear that categories are, well,
categories of *items*.

Update redirects. When a user deletes a category, redirect them back to the
/item/ page. This is in line with the new breadcrumb links.

Update wording in the priority categories sidebar:

    * "category name (details)" -> "category name"
    * "Available" -> "In stock"
    * "Next due out" -> "Next out"
    * "Next due back" -> "Next back"