[REF] website_slides: apply linting and cleaning to elearning lesson pages and widgets

Purpose of this commit is to perform some linting and cleaning in lesson widget
JS and its related css. Notably for fullscreen widget of eLearning. Containing

 * better naming of css classes;
 * deleted sidebar file as it is now included directly in the fullscreen
   widget;
 * removed unused css;
 * apply JS linting;
 * clean some controllers;

Commit linked to task ID 1942852 and PR #31394.