[REF] website_slides: handle uncategorized slides in training course view

Purpose
=======

This commit adds the management and display of uncategorized slides
for both the frontend publishing management and the fullscreen
course views.

We add a "Uncategorized" section on top of all the categories that is
hidden for the student if it's empty.

Course publishers can drag and drop slides from and to that
new "Uncategorized" section to easily reorganize the course.

Commit linked to task ID 1942852 and PR #31394.