Use roster identifier in downloaded repository names

[jtppragg]

When downloading all the students' repositories for a given assignment, it is more useful to name the directories after the student's roster ID. Currently, the name of the student GitHub account is used as the directory name and we have to manually map that names into the ID we use to track and evaluate them.

I similar issue was opened (and fixed) for GitHub Classroom: github-education-resources/classroom#1877

[TrutzX]

+1