update mark_student_can_skip_entrance_exam api to DRF

[awais786]

Existing code

Postman testing

1. send post request to this URL ( user your course id )
2. pass the form-data as unique_student_identifier=admin

expected result
{ "message": "This student (admin) is already allowed to skip the entrance exam." }

end to end testing

1. Go to this instructor->student-admin page
2. Pass the username in Adjust a learner's entrance exam results input box
3. press the Let Learner Skip Entrance Exam button.