You're no longer restricted to just one, special-case admin thumbnail. Make as
many as you want by adding AdminThumbnailView properties to your model and
including them in your admin class's `list_display` tuple. You can also provide
a custom template. Note that (because this change introduces templates to
imagekit), imagekit is now required in INSTALLED_APPS.
Ideally we could get this stuff out of the model, but we'll have to look into
whether that's possible without making things really complicated.
