Fix for an error related to endless recursion
Dev Notes:
The student gets the passing grade
-> CourseGradeFactory sends the `COURSE_GRADE_NOW_PASSED` signal after the grade calculation
-> `listen_for_passing_grade` receives the signal and calls `generate_certificate_task`
-> `generate_certificate_task` calls `_generate_regular_certificate_task`
-> `_generate_regular_certificate_task` trying to get grade by calling the `_get_course_grade`
-> `_get_course_grade` calls `CourseGradeFactory().read()` but I have persistent grades off, so actually that ends with `CourseGradeFactory().update()`
-> `CourseGradeFactory().update()` sends the `COURSE_GRADE_NOW_PASSED`
And so on
c4bc6e29ea