feat: Add a role management API for both courses and organization-wide roles

[OmarIthawi]

Faris will provide us with a design so we make the API for.

But generally the API should allow the following:

GET /roles
user_id:

POST /roles
org: (double checked for secuirty reasons)
course_id: (optional, if we need a course-specific role)
user_ids: ['test@example', 'username123'] (multiple users at once)

DELETE /roles
By role ID

DELETE /roles
by user ID

---

Supported roles:

• Site Staff (مدير الموقع)
• Site Instructor (طاقم الموقع)
• Course Community TA (مساعد أستاذ لشؤون المتعلّمين في المقرر)
• Course Group Community TA (مساعد مدرس لمجموعة المنتدى)
• Course Discussion Moderators (مشرفو منتدى نقاش المقرر)
• Course Data Researcher (باحث بيانات المقرر)
• Course Discussion Admins (مشرفو نقاشات المقرر)
• Course Beta Testers (مختبرو نسخة المقرر التجريبية)
• Course Staff (مشرف المقرر)
• Course Instructor (طاقم المقرر)
• Course Creator Role (منشيء مقررات)

Notes about roles:

• Staff vs Instructor: Staff can add/remove instructors, but not the other way around.
• Course-specific and org-wide roles are the same but the difference is the course_id property is null for org-wide roles.