[Course Import/Export] Warn about Group Discussions/Assignments on Import

[James_Kocher_UF]

When copying content from other courses, any group assignment/discussion gets assigned to "project groups" as the default. It would be nice to have a warning after import to let the user know what assignments/discussions are set to groups to avoid any confusion once the course starts. 

We get countless support calls about copied courses where instructors forgot to update the group set assigned and experience errors in Speed Grader.