Allow displaying the description at the course main page
Fix the grade displayed at the course main page
Support refcourse set to zero or non-existing course

This should handle various situations like deleted referenced course.
Explicitly unreferenced subcourse (i.e. with refcourse set to zero) will be
needed to support restore properly (we must not transfer refcourse to another
site).

General code cleanup has been done while working on this.
Implement the backup and restore support
Display the fetched grade at the subcourse view page
Fix log display
Drop the 'grade' field from the 'subcourse' table

We have nothing to be stored here, grades are copied in the gradebook.