@mbertheau raised a good point that rebinding the image formsets as
added in 0b373df hides any errors in the image formset.
0b373df5ab (commitcomment-5368878)

This fix undoes that change and instead alters behaviour of the
ImageInput widget to not display images that were never persisted to
disk.

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

Fixes #1134.

(cherry picked from commit 375a8f4515)

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