[FIX] website_slides: fix various quiz issues

This commit fixes a few issues with quizz

  * change background-color to white in fullscreen mode;
  * prevent going up to the top of the page when selecting an answer;
  * highlight selected answers in fullscreen;
  * fix the display of already completed quizzes

Commit linked to task ID 1978530 (fixes linked to internal testing).

closes odoo/odoo#33140

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