[MERGE] website_slides: improve layout of courses main pages and apply various fixes

Purpose of this merge is to provide several fixes related to new elearning
application [1]. This is second batch of fixes [2]. Those are mainly fixes
and improvements for front-end templates: layouting, clean design. Some
other fixes are also included, notably links and search capabilities.

See sub commits for more details.

Thanks to @awa-odoo @jem-odoo @stefanorigano @qmo-odoo @dbeguin
for their work as most of code come from cherry-picks.

Merge linked to task ID 1941250.

[1] eLearning feature: see task ID 1902304
[2] First batch of fixes landed at 5cec06b179f3dfe48c091f627d9e9c3255c30438

closes odoo/odoo#31317

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