Assignments: allow comments between teachers

[ywarnier]

Actual behavior

In the assignments correction, all teacher comments are visible to students.

Expected behavior

Sometimes, teachers want to be able to comment on an assignment submission without that comment being visible to the student (but they can be viewed between teachers of the course).
To enable this feature, comments should be marked "private", so a new field will be necessary in the table that stores the assignments corrections (c_student_publication_comment).

Apparently this feature was available in the old Claroline (1.11), and it is being missed by those who changed from Claroline to Chamilo.

[ywarnier]

Relates to the tracking page developed for Encora, but not the same.