Waheed/ora72 consistent rubric scores

Bug: https://edx-wiki.atlassian.net/browse/ORA-72

The ML graders calculate the composite score independently of the rubric scores. This means in a lot of cases composite score is not equal to sum of rubric_scores. While the composite score is a more accurate grading of the essay this causes the discrepancy between the scores of the progress page and the ones in courseware. This set of changes in the case of ML grading set the score of the attempts in the child_history to be equal to the sum of the rubric scores.

I have also added some tests for this combined open ended xmodule rubric scores consistency.