Celery requires all tasks to be in a tasks module. Also, the task has been updated to use default values if the the course structure generator encounters a block without the graded or format fields.

CourseLocator is not JSON-serializable. Passing course keys as Unicode instead when scheduling tasks to update course structure.

Sorting on unicode value of course ID since CourseLocator objects cannot be compared to SlashSeparatedCourseKey objects.

Updated filter to ensure only course descriptors are returned.