[IMP] website_slides(_*): improve demo data

Purpose of this commit is to improve eLearning demo data. Notably links
(links to external resources on a given slide) and quizzes (allowing to
gain karma) are added. Some slides are added. Demo user interaction with
eLearning is also added, such as membership and votes. Finally a channel
is specifically kept specific to 1 single slide of type certification as
its flow has to bypass some screens when selling pure certifications.

Commit linked to task ID 1941250 and PR #31453