Use staticfiles_storage to generate static files urls for widgets.

[n6g7]

Hi,

Currently the urls for the widgets assets aren't processed by the project's defined static file storage (STATICFILES_STORAGE in the settings) and so, when using a custom static file storage (like django-s3-storage), the generated urls results in 404.

I sent these file paths through django.contrib.staticfiles.storage.staticfiles_storage.url to address this issue.

This fixed a bug I had where the assets for the admin widget would point to /admin/app/model/id/django_markdown/... instead of https://s3-eu-west-1.amazonaws.com/bucket/static/django_markdown/... (I use S3 storage) and thus the widget would render as a simple textarea.