[FIX] website_profile, website_slides: clean user avatar controller and add one for slides

Purpose of this commit is to

 * clean user avatar controller and its calls;
 * extract default image computation;
 * add a controller to get slide image even when not having access to the slide
   as all slide name and images are displayed even if not available to the
   current user;

Commit linked to task ID 1941250 and PR #31133.