[MERGE][IMP] website_slides: improve inclusion of gamification and ranks

Purpose of this merge is to improve inclusion of gamification and ranking
in elearning new main pages [1][2]. Ranking and profile information, links
and templates are added.

Some fixes are included in this merge, notably statistics computation, better
slide image fetch and demo data. See sub commits for more details.

Commit linked to task ID 1941250.

closes odoo/odoo#31133