[MERGE][REF] website_slides: clean access and add membership on channels

Purpose of this merge is to clean access rights on slide models and give
less access to public / portal / internal users. Website publishers and
editors have rights to edit slides content and manage channels.

Another purpose is to clean and refactor access options on channels and slides.
Access to a given slide depends on the current user being a member of the
slide channel. A channel can be either public (joined easily) or on invitation
only (manual addition of members).

Slide can be flagged as preview, meaning they are accessible without any
concern about channel membership. It is used as teasing or free promotion
of a channel.

Publish flag is orthogonal. Non published slides are not visible to everyone
except website content editor. It allows to work and improve slides without
displaying them to members or public people.

Slide upload access is a bit cleaned. A can_publish field is added controlling
who can publish slides. Currently website publishers are allowed to publish
content. Not publishers people allowed to upload cannot publish their own
content. It stays in a submitted non published state.

Base for tests are also added. More of them will be added in future commits.

This merge is related to task ID 1937411 and closes PR #30847. More
generally this merge is linked to ongoing tasks

* task ID 1902304 (main eLearning task) PR #29876;
* task ID 1922159 (new user profile and gamification) PR #30514;
* task ID 1936153 (new homepage for slides) PR #30770;
* task ID 1937160 (payment flow and integration with ecommerce) PR #30914;

This merge is therefore not functionally complete itself and serves as a
preparation for other incoming commits. See sub commits for more details
on what has been done here.

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