Course Primary Key Refactor

[haram082]

1. Connect course reviews to “course_slug” within code base
2. Adding “course_slug” to course_reviews (making it unique)
3. Update backend routes to generate based on course_code
4. GET request courses/{id} extends to also take in coures/{course_code}
5. Both for courses and course reviews
6. Frontend: extend to also take in courses/{course_slug}
7. Future: remove Id, course code from courses