[IMP] website_slides_{*}: improve backend UI, views and user actions

Purpose of this commit is to perform a back2basics on eLearning backend
views. It helps users in their daily use of eLearning and improve UI.

Containing

  * channel kanban view tweaking;
  * channel form view: some spacing and renaming improvements. Also set
    advanced field (new content email, share template) as group no one;

  * slide kanban view tweaking;
  * removed sections from kanban and list view for contents;
  * contents form view: various improvements, with notably

    * removed field website_url;
    * set publishded date is now readonly, tracked and in group_no_one as
      it is automatically set when publishing the lesson, and people should
      not update it manually;
    * removed the sum "Content" and replaced it by a stat button
    * reword quiz constraint error messages;

  * rating views improvements:
    * revamped the kanban view of rating that were a bit broken;
    * added a pivot view in Reporting/Reviews;
    * updated graph view in Reporting/Reviews;

This commit also provides some usability fixes

  * prevent sending email if new content is category;
  * email: Prevent displaying an image if there is no image;

LINKS

Task 2049640 (website slides back2basics and fiximp)
Task 2058595 (eLearning v13 testing)