implemented enroling individual users to a course

This allows instructors (not TAs) to enrol small number of users to their
course without having to deal with setting up a CSV file.

- the feature has been enhanced to allow enrolment of any course roles
instead of just instructors
- the list of users that can be search include every user in the system
- created two new API functions and their tests to implement this feature

Furthermore:
- change the system role column of the class list to course role
- enhanced the class list to be able to be sorted by first name, last name,
student number, display name, email and course role
- fixed ordering the student filter in the evaluation results page
filter out instructor answers when the group and student filters

are used