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 
I worry about allowing unicode word characters in here. That's probably fine for most filesystems, but limiting strictly to ascii alphanumerics would be more universally applicable. If the filesystem doesn't support unicode characters, what would the failure mode be?