This allows admins to:

* Preview emails in the dashboard
* Send preview emails
* Save emails

Fixes #359

The previous UPC validation in ``ProductForm`` broke updating the
a product. I realised this while digging around to fix the broken
``StockRecord`` issue that broke the CI build.

The validation now uses Django's validation of ``unique`` fields
instead and works.

The ``StockRecord`` issue was related to creating a form in the
update view providing stock record data in POST with ``partner``
empty. I corrected that and added a test case for it.

Attempting to create a new product with a UPC that already exists
raises an ``IntegrityError`` instead of handling the ``ProductForm``
as an invalid form with an appropriate ``ValidationError``.

I fixed that by adding a ``clean_upc`` validation method to the
form by checking for an existing product by UPC. I am not sure if
that is the most efficient way to validate this. If there's a
better alternative, I am happy to change it...and learn something
:)

**Note:** I switched the functional tests in ``product_tests.py``
to use ``WebTest`` while I was at it.

* Tests now split into more coherent modules
* Added IDs to forms to help WebTest
* Some refactoring along the way

The validation on the stockrecord form needed to be removed to allow
group products to be created, which don't have stock records.

They now must have a title

Also split them into unit and functional tests.