[MERGE][ADD] website_slides_sale: allow to sell slide.channel

Purpose of this merge is to be able to sell courses [1] when using the
slides / eLearning platform.

This commit adds sale capabilities on a slide.channel. A slide.channel can
now have the 'payment' visibility, that requires a 'product_id' configured
on the channel.

When a customer purchases a product linked to a channel, he is added to the
members of the channel (see slide.channel.partner_ids) when his order is
confirmed.

This merge is linked to task ID 1937160 and PR #30914. Future tasks will
improve homepage of channels and clearly show public, private and payment
based channels [2].

[1] see task ID 1902304 (main eLearning task) PR #29876;
[2] see task ID 1936153 (new homepage for slides) PR #30770;

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