Admin can assign users to the "Instructor" role

**Description:** Admin have the ability to assign users to the "Instructor" role.

**Estimated time/effort**: 10hrs

**Estimated size/complexity**: M

**Dependencies:** User roles should be implemented

**Team assignment:** @568cats 

**Milestones:** Should be completed by the end of Sprint 2

**Acceptance criteria:** An "Instructor" role is visible by admin under users who are assigned as admin