Fix 500 error in transcripts.

When ID was removed from unit html (in https://github.com/edx/edx-platform/pull/1680) , 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 (Can't import or add transcripts for video)

In longer term - @cahrens what should be changed in transcripts files to support (https://github.com/edx/edx-platform/pull/1680) change?

@polesye can we add tests for this case?

@lyla FYI