[REF] website_slides: improve elearning backend usability and onboarding

PURPOSE

Improve elearning backend usability and onboarding

SPECIFICATIONS

Improve layout of email send when invite. Do like a SO, add the join button
in the notification email and not in the composer itself. It required to
create a new notification layout specific to that invite wizard.

Improve channel form view: remove statistics as they are already displayed
in various places, add a stat button for content/visits, better display of
karma (split gain from karma-based access rules).

Automatically publish channel's product to avoid de-synchronization of publish
flags.

Remove unused channel_url field on wizard.

LINKS

Task ID-2241513
PR #55698