Fix bug affecting video transcripts in XMLModulesStore courses

This corrects a bug discovered on staging that was introduced by https://github.com/edx/edx-platform/pull/3213

It now checks for the presence of a modulestore in the run time and uses the data_dir to create the static path if it isn't there (the case with XMLModuleStore courses).

suggested reviewers: @auraz and @poleseye since they reviewed the original transcript changes.