Add DropStudent endpoint.

[krashanoff]

• Professors can drop any student.
• Students can always drop themselves.

POST /class/<class_id>/drop

Drop the student with ID id from your class with ID class_id. Any student can
drop themselves from a class, and a professor can drop anyone from a class except
themselves.

Request Body

{
  "id": "ID"
}

Response Format

No data is returned with this endpoint.

Status Code	Semantic
200	OK
400	Bad request (see format)
401	Unauthorized
500	Server error

[krashanoff]

Force-merging this since Cindy is using this behavior already.