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.
Use `value = unicode(value)`, please.