Improve Group assignment submission exporting

The general behavior is tested using PHPUnit tests.

However, the download itself cannot be tested using automatic tests.

Prepare manual tests scenarios

1. Create a course with 3 students and 2 groups
2. Assign students 1 and 2 to group 1

Manual test 1: one folder per student

1. Create an assignment with the default options (Students submit in groups to NO)
2. Log in as the 3 students and do a submission with each one of them
3. Log in as admin and go to view submissions
4. Click on the "Download all submissions"
5. Open the downloaded zip file
   1. Check there is a folder per student with the submission in it

Manual test 2: one folder per group

1. Create an assignment with:
   1. Students submit in groups to YES
   2. Require group to make submission to NO
2. Log in as student 1 and 3 and do a submission with each one of them
3. Log in as admin and go to view submissions
4. Click on the "Download all submissions"
5. Open the downloaded zip file
   1. Check there is a for group 1 with the student 1 submission
   2. Check there is a folder for the Default team with the student 3 submission