[FIX] website_slides: fix various issues to eLearning platform

Including

* change h4 to h1 on the slide page;
* add the rel='canonical' link header on the fullscreen view in order to
improve SEO and links;
* add space between quiz questions;
* remove social share statistics as they are broken and not really useful;
* moved tabs before the Quiz on the slide page;
* allow to modify slide in backend by removing some readonly attributes;

Linked to task ID 2010024
Related to PR #34176

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