Filenames can easily get longer than 100 characters, and that causes
Django to fail hard (I think with a DatabaseError), because it's
currently not validated (see Django #11027)

This fix increases the character limit to 255 chars. Going much higer
can cause trouble with MySQL.

Fixes #625. Thanks for reporting go to @soloweb.

They need registering using oscar.apps.offer.custom.create_range which
will create the model instance with the correct proxy class path.

After that, they will be available to see in the dashboard (but not
editable) and can be used in offers.

Fixes #385