Describe the bug

Related to #11763. Issue more noticed #11792. When PR#11792 was merged, all the withdrawn students were hidden on the details view of TAGrading. However, the statistics still counts the withdrawn student's submission as a valid submission, despite the TA's most likely never seeing the student to grade.

In my opinion, we should configure statistics in a way such that withdrawn students are excluded by default, and another option can be added if we want to factor in the withdrawn students in the calculation.

Expected behavior

We should not include the withdrawn students in the statistics.

To Reproduce
There are two main reasons why this is useful:
in the details page, where TA's often look at statistics to see whats doen
In the home/course page, where the button will still be red and display that TAGrading is not done, despite the TA's not needing to grade that particular student.

Reproduction: Student makes a submission, withdraws from the course, see two errors above

Configuration

• OS: [e.g. Windows, Linux, ...]
• Browser [e.g. Chrome, Safari]

Screenshots

Additional context