Fix 500 error in transcripts.

When ID was removed from unit html, transcripts were left broken, as they use id (location) and get_item(item_location) to obtain transcripts files.

This is CAT-1 blades bug: https://edx-wiki.atlassian.net/browse/BLD-530

In longer term - @cahrens what should be changed in transcripts files to support this change?