Add entries to `urls.py`

Add entries for interacting with categories of items, such as
`/category/<id>/update-form/`.
Add barebones views functions.
Add class ``Category`` to models.py

The new class is an integral part of the "priority categories" feature. It
allows users to create pre-defined searches based on sets of tags.
Add CategoryFactory
Fix up factories.py

Make the `factories.item_tags()` function return a list of Tag objects, not Tag
IDs. Adjust the one usage of that function to match the new behaviour.

Write a docstring and some doctests.

Make pylint ignore a warning.
Fix bug in CategoryFactory.tags()

Find bug, write doctest, fix bug.