LDEV-5111 Display groups instead of students in chart when needed
If Assessment activity uses a leader, then only he gets his answers auto
saved. He counts as one student. But when he finishes the activity, his
answers get copied to other group members. Suddenly instead of 1 student
we get, say, 5 with same answers. This makes the answered questions
chart go crazy.
Now when leader selection is in use, group count is displayed instead of
student count in the chart.
