Allow HTTP GET /category/<id>/ to succeed.

Make it possible for a client to issue an HTTP GET request to /category/<id>/
URLs. In other words, a client can now fetch information about a particular
category. This change makes the category* views more in line with other families
of views, such as the item* and tag* views.

Update various views to point to issue HTTP redirects to this new view, and also
update tests to match.
Add HTML UI for /category/* pages.

UI is not yet done. In particular, `base.html` needs to be updated to show a
proper list of priority categories and their statuses. However, all other pages
are basically functional.