Fix export course with old ORA components.

STUD-2096

Courses which have old ora components are not able to export because BlockUsageLocator was not handled in EdxJSONEncoder.

Peer grading interface save doesn't work link_to_location is given in settings and edit is not working for those peer grading interface components which already have link_to_location in metadata.
This should be:

    if isinstance(obj, OpaqueKey):
        return unicode(obj)

Thoughts, @dmitchell?