[MERGE][FIX] website_slides: clean course and lesson pages layout and fix various small UX issues

Purpose of this merge is to at last perform a cleaning of layout and design for
eLearning pages still not completely updated with magic layout touch.

Impacted pages
 * course (channel) main page: better top and banner, better navigation.
   Improve training content list display, clean documentation content list
   display and navigation;
 * lesson (slide) main not-fullscreen page;
 * lesson (slide) fullscreen page / widget;
 * homepage: clean navigation;

This merge also contains various fixes and UX improvements for fullscreen
lesson display as well as user profile page. Thanks to @stefanorigano and
@qmo-odoo for their input.

This merge closes tasks 1941250 about fixes and layouting of new eLearning
application. Thanks for your attention.

closes odoo/odoo#31977

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>