[IMP] website_slides: new homepage and main channel page

This commit proposes a new homepage for slides module evolving towards
an eLearning platform. 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;

For more details about specifications, send an email to aware people that
will be able to send you mockups. Or see the related task. Thanks to
@qmo-odoo who helped developing this homepage based on its work.

This commit is linked to task ID 1936153 and PR #30770.

Co-Authored-By: qmo-odoo <qmo@odoo.com>
Co-Authored-By: Thibault Delavallee <tde@odoo.com>