[C2] Course category : there is no option in the interface to create and manage sub categories

[NicoDucou]

On the page main/admin/course_category.php you can manage categories and create a new category.
When entering in a category if you create a new category there is in the url the id of the parent (main/admin/course_category.php?action=add&id=5) but once you create the category the parent category is not registered in the database.

Also when editing a category (main/admin/course_category.php?category=&id=6&action=edit) the parent field is missing to be able to select the parent category.

Once the assignation of a category works, check that it appears correctly on the main page (main/admin/course_category.php) and when entering the parent category.