message: remove some props from exported course
diff: @@ -94,6 +94,9 @@ courseSchema.methods.exportJSON = async function () {
  delete obj.updatedAt;
  // custom properties
  + delete obj.accessKey;
  + delete obj.active;
  + delete obj.whitelist;
  delete obj.students;
  delete obj.courseAdmin;
  delete obj.teachers;
  