Add parameters to the students API endpoint to allow filtering

[marcospri]

For:

• https://github.com/hypothesis/lms/issues/6393

• Filter by course_id

• Filter by assignment_id

Testing

• Fetch course and assignment IDs based on:

http://localhost:8001/api/dashboard/courses http://localhost:8001/api/dashboard/assignments

• Play around constructing URLs to filter them like:

http://localhost:8001/api/dashboard/students?course_id=1830&assignment_id=349