added a group filter to the evaluations results page

- this allows TAs to filter the student list with groups
- the groups filter is not available if there are no active groups
in the course. This is to reduce confusion to user that does not use
the group feature
- I've also modified the classlist api to return students with the
CoursesAndUsers dataformat. This is so that I can use the user attribute
that GroupsAndUsers also have to reduce the complexity of the code