The widget returned Python 2's string type, but should return unicode.

Fixes #1134.

It doesn't exist in Django 1.4. Easiest fix is to just backport it.

We define a custom select widget which allows certain choices to be
marked as disabled.  We use this to include unavailable variants in the
add-to-basket dropdown.

The entire image handling in the dashboard isn't very pretty. But at
least we're not using full-size images for this.
We also can't assume that just prepending MEDIA_URL is the correct URL
for every image (e.g. not when using S3 backends). Passing it through
Sorl gets us the correct URL for free.

This allows admins to:

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

Fixes #359

Includes:
* Reworked CSS
* New image field for product form

Reworking of #326