Added get_top_submissions to submissions API

Added a new API to the submissions module which gets the top
submissions for the assessment
It would be clearer to change the function signature to include only the arguments you plan to use, then provide them explicitly to the `filter` method.
```
def get_top_submissions(course_id, item_id, item_type, number_of_top_scores):
    try:
        student_item_models = StudentItem.objects.filter(
            course_id=course_id,
            item_id=item_id,
            item_type=item_type
        )
```