StudentModule specifies student, course id, and module state key must
all be unique together. The reset code in module_tree_reset was calling
get without specifying course_id which means if two courses (with
different course ids) share the same users and module state keys then
calling StudentModule.objects.get with only the student and module state
key raises an error for users that tried both courses. Specifying all
unique_together fields fixes the issue.
