teachers panel: results view should show statistics

[meling]

When in results view, there should be a column for slip days. And there should also be a separate statistics panel above the results list that shows:

• slip days spent in total, on average, and the distribution over the days.
• how many have been approved for each lab
• the average grade (in %) for each lab and in total.

[meling]

We should also show how many students have signed up on autograder to make it easier to compare with the numbers in ITLs list of students.