Rewrite PrivateFileField.generate_filename() for better Django 2 compatibility.
Rewrite PrivateFileField.generate_filename() for better Django 2 compatibility.

- Support upload_to parameter (#3, #13)
- Support upload_subfolder returning strings instead of lists (#5, #11)
- Avoid restoring the deprecated get_directory_name() and get_filename()
  functions that were deprecated as of Django 1.0 and removed in Django 2.0
  Instead, generate_filename() was updated to support the recent Django
  behavior

edoburu/django-private-storage