Rename session variable: 'form' to 'form_data'
Update docs
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.
Fix up factory function `item_tags()`

Make function return a list of Tag objects, not a list of Tag IDs. Accordingly,
update the unit tests for this function, and change the one usage of this
function to compensate for the change.
Add class CategoryFactory and helper function
Make truthiness check more strict

Do not simply check to see if a particular object evaluates to True. Instead,
check whether that object is None. Change ordering of if/else statements to
accomodate this new, more strict check.