New fixed group for students

[sarahjeansweeney]

In order to support more nuanced licensing agreements for vendor supplied materials, users need to be able to restrict access to certain files to just students and faculty. Our current fixed groups include "Northeastern - All", "Northeastern - Staff", and "Northeastern - Faculty". We've been relying on "Northeastern - All" for materials that can be accessed by students, but "Northeastern - All" can include alumni, which is a group that is often explicitly prohibited from accessing the material. Adding one more group for "Northeastern - Students" will allow users to set permissions that comply with vendor licenses.

n.b. unless the upload user is identified as a student by shib, someone with admin user permissions will still need to select this group on behalf of the user uploading the material. the 2.0 spec addresses this