[MERGE][IMP] website_slides: introduce a new home page for slides

This merge proposes a new homepage for slides module evolving towards
an eLearning platform. This merge closes task 1936153 and PR #30770.
More generally this merge is linked to currently under heavy work slides
module update to eLearning [1][2].

It includes

 * a new main page displaying top courses / channels. It displays 3 most
   popular and newest channels as well as its ongoing courses (if logged).
   Achievements and karma update done by eLearning community allow to
   give a gamification look and feel;
 * an 'All' page displaying all courses / channels. Tag groups and tags
   allow to search / filter displayed courses;
 * a new pimped and revamped display for main course / channel page;
 * new demo data for slides and small addition of demo data in bridge module
   with website_sale and website_forum;

Some features may not be completely working as other tasks will continue to
improve the display, notably new slide display [1], some forum / slide
link improvements [3] and certification integration [4]. Some code cleaning
will come in future update as this merge allows to unblock those other tasks.

For more details see sub commits. And thanks to the RD Fun SM Team. Much
fun, much SM.

[1] main eLearning task: task ID 1902304 - PR #29876
[2] new user profile /gamification: task ID 1922159 - PR #30514
[3] forum and comments upgrade: task ID 1940516 - PR #30514 and #31097
[4] certification inside eLearning: task ID 1940360 - PR #31060