SHOP-1034: Allow default thumbnails to be set

Set the field's name to the 'default' attribute if it's been given, so
that the field is always populated with a default file from which to
make thumbnails, if a default was provided to the field.

extra_thumbnails should default to {}, not []
Clear up flake8 errors
Merge pull request #1 from giftig/SHOP-1034

SHOP-1023: Allow default thumbnails to be set