Make sure temporary course directory uses safe characters.

Code to convert course_ids to filenames is handled outside the opaque keys library.  

Here is one relatively insignificant place, but one that is needed to support export for researchers.  Hopefully this matches the general we want to take in the LMS and analytics.  @nasthagiri @cpennington @clintonb 
As a case in point, my local filesystem currently doesn't support unicode.