add another account setting which the student's academic id

[williamstein]

This would be an optional field that students can configure in account prefs (and maybe elsewhere). It would be labeled "Student Id". Instructions would then see this as a new field in course management and grade export.

[williamstein]

This should be something that instructors should be able to override and/or upload to the .course file, similar to what instructors can do right now with their first/last name.