API endpoint for batch fetching student points

[no-comma]

A+ has the following API endpoints for fetching points:

GET /courses/<course_id>/points/ - for fetching a list of students
GET /courses/<course_id>/points/<user_id>/ - for fetching the points of an individual student

In Grades, we want to fetch points for all students of a course. The only way to do that currently is to first fetch a list of students, and then for each student, individually fetch their points. Courses can have up to hundreds of participants, so this means there will be hundreds of API calls.

An API endpoint for fetching points for all students at once would be helpful. For each student, we need:

• Their full points
• Their points for each module
• Their points by each difficulty level

[mikaelGusse]

To me it looks like GET /courses/<course_id>/points/ returns the exact same list as GET /courses/<course_id>/students/ currently. With this PR#1372 I was able to fix it with a very small change.