Fix transcripts in XML course when no static_asset_path is set.

Case where static_asset_path is not set in XML course.

In such case use data_dir.
Issue: https://openedx.atlassian.net/browse/BLD-67

@cpennington @carsongee 
> This adds a dependency of the video module to django, which is really not ideal. I know there are already a few such dependencies in the video module, but I'd like to avoid adding any more if possible.

Removed dependency. Use `descriptor.modulestore`