Now that field is not nullable.

Recent commit regarding null=True on CharFields (44da815) broke
importing the offers fixtures because of a field being null.
The Travis builds did not fail because loaddata wrongly returns an exit
code of 0: https://code.djangoproject.com/ticket/20538

Fixes #655

Was getting pretty bored of creating one everytime I want to test the
offers dashboard.

Fixture is loaded as part of 'make sandbox|demo' targets.

Learnt the hard way that promotion fixtures will have different content
type IDs in each environment.

This allows admins to:

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

Fixes #359

Was an "image-not-found" image from another project, where I originally
harvested the testing images.

Also, sym-link in Oscar's image-not-found image to the assets/media
folder.

Remove old recreatedb script in favour of using the makefile.

Update makefile and docs to reflect this change.