[REF] website_slides_forum: improve integration into eLeaning application

PURPOSE

eLearning should have its own application in manager. Indeed this is becoming
a big application and having it embedded inside Website application is not
enough anymore. It should also hold updated and easy-to-use menus,
navigation, actions and views in backend.

SPECIFICATIONS

Add menu after Courses

  * Forum
    * Forums: add columns to list view: Posts (questions count), Views
      (sum of views of each post), Answers (sum of child_count on each post),
      Favorites (sum of favourite_count on each post)
    * Posts: add columns to list view: Views / Answers / Favourites

Add menu entry inside Reporting
  * Forum: graph view on forum.post model
      Area chart: # Posts created by months, color=forum

LINKS

Task 1978729
PR #35061

Co-Authored-By: Thibault Delavallée <tde@odoo.com>
Co-Authored-By: Jérémy Hennecart <jeh@odoo.com>