Simple group account

When enabled in the Project settings, teachers can create special "group" accounts. These accounts are essentially student accounts, except that they:
 - Are restricted to logging in only during school hours (defined in Project settings)
 - Are sorted to the top of the class roster and student lists in reports

I don't know if Scott had intended for this to get merged into master, but since it can be turned off and on in the project settings, it shouldn't hurt anything.