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. Uses `descriptor.modulestore` now.

>I wonder whether it would be possible to make the LMS do the same rewriting of 30* Location headers that it does of links in html content, for instance. That way, all of this knowledge would move out to the runtime, and wouldn't be the responsibility of the video_module at all.

@cpennington Let`s leave rewriting stuff for another PR.