[FIX] website_slides: fix placeholder and kanban images
[FIX] website_slides: fix placeholder and kanban images

PURPOSE

Test frontend and UI tools of eLearning.

SPECIFICATIONS

This commit fixes issues spotted when testing eLearning in backend

  * placeholder image was referring an unexisting image;
  * kanban was still using image fields prior to some image.mixin refactoring;

LINKS

Task ID 1937768
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo