Add api endpoint for list of student points in a course

[mikaelGusse]

Description

What?

Fix issue where api endpoint GET /courses/<course_id>/points/ was showing the same list as GET /courses/<course_id>/students/. Now endpoint GET /courses/<course_id>/points/ gives the user a list of all students in a course and their points in the course.

Why?

Allows for easier parsing of points for Aalto Grades.

How?

Changed which list is given to the listserializer

Fixes #1360

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