Issue 1001: added code to update group coverage when randomly assigning graders to criteria

https://github.com/MarkUsProject/Markus/issues/1001

In graders_controller.rb, there is no update action in the randomly_assign_graders_to_criteria method to update group coverage (such action exists in add_graders_to_criteria method). Therefore the group coverage remains unchanged when randomly assigning graders. The required update code is added to randomly_assign_graders_to_criteria method.