[FIX] website_slides: fix some statistics and publication issues

This commit fixes various issues introduced by recent commits in slides

 * fix slide type computation: using a dict in a fromkeys makes a shared
   object for all keys. It is better to fix that behavior;
 * correctly display number of course steps in elearning homepage. Currently
   number of steps of a course is simply the length of its one2many;
   holding slides. However this one depends on access rights which give funny
   results like a course with 2 steps for public users and 7 for admin;
   Right computation is: number of published lessons, whatever your access
   rights are. This is already computed as total_slides;
 * people that cannot publish should not be able to unpublish;

Commit linked to task ID 1941250 and PR #31133.

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