[MERGE] website_slides: clean controllers and apply various fixes

Purpose of this merge is to perform some code cleaning in eLearning
controllers and demo data. There are also some fixes coming from first
user testing of eLearning. See sub commits for more details.

Merge linked to task ID 1941250 ("fix all the eLearning").

closes odoo/odoo#31512

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>