Fix form serialization issue

Django 1.6 now uses JSON to serialize objects by default, not pickle. This fixes
a security issue, but at the cost of being able to conveniently serialize any
Python object. This commit fixes one of eight form serialization issues detected
by the test suite.
Fix serialization issue.

Six serialization issues remain.
Fix serialization issue.

Five serialization issues remain.
Fix serialization issue.

Four serialization issues remain.
Fix serialization issue.

Three serialization issues remain.
Fix serialization issue.

Two serialization issues remain.
Fix serialization issue.

One serialization issue remains.
Fix serialization issue.

Zero serialization issues remain, according to unit tests. Looking through the
codebase shows that more serialization issues exist that simply haven't been
detected by the unit tests.
Reformat code

Squish several sections of code onto fewer lines. All squashed sections still
run under 80 characters long.
Add unit test and fix serialization issue.

The chunk of code that contained the problematic serialization code was not
previously covered by unit tests. This commit adds a relevant unit test, then
fixes the issue.
Update factories.

Create class ``LendNoteFactory``. Move factories further down in file, to
prevent issues with "undefined functions". Add helper function
``lend_note_is_complaint()``.
Write an enormous number of unit tests.
Satisfy unit tests.
Update module `tests`

Update module to comply with the unit testing framework's API [1]. This means,
among other things, changing the function `suite()` to `load_tests()`.

Also, due to the upgrade to Django 1.6, it is no longer necessary to create a
suite of unit tests from this module. The test runner automatically discovers
all unit tests inside files named test*.py.
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.