course leaders by points_scored

@mattdrayer added api to get leaders in a course by points_scores. It'll support MCKIN-1516
We also need to consider that there are different perspectives on what a 'leader' is for a given course.  For example, we have leaders by grade, as well as by progress.  So we should think about how to support these two use cases as well as future ones in terms of URI(s) and response(s)