[FIX] website_slides: fixes some display issues in eLearning frontend

Fix some display issues in eLearning

  * fix the share button in documentation. It was not working as model was
    not correctly placed;
  * fix top3 users banners broken in mobile;
  * fix left menu of fullscreen being behind content on mobile (image, video);
  * removed upload button in training channel aside as there are already
    buttons available for that purpose;

Task 2049640 (website slides back2basics and fiximp)