update_forum_role_membership api to DRF

[awais786]

Verify via postman

1. Send post request to this URL
2. form-data
   unique_student_identifier: username
rolename: pass any from the list [Administrator, Moderator, Group Moderator, Community TA]
   action: allow or revoke

expected result
{ "course_id": "course-v1:edx+cs222+2311", "action": "allow" }
Verify via instructor dashboard

1. Go to this page
2. select the role from drop down and press add. It will appear in the Discussion Moderators list.
3. Now press revoke link. It will remove the user from the list.