#42 Option to let "Upload Student Roster" replace entire roll

[Tony Allevato]

When uploading a CSV file of a class roll to Web-CAT, the user should have the option of letting the uploaded file *replace* the entire course offering's roster instead of just adding to it.

This would be useful near the beginning of a term when people add/drop classes frequently, making it easier to filter out students who were in a course at the beginning but soon dropped it.