[IMP] website_slides: clean ACLs and refactor access options

Purpose of this commit 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.

Technically a new model is added to model membership on channels. It is
a decorated m2m between channels and partners; on this m2m the state of
membership is added. In a near future additional information will be stored
on this model, like completion of channel or certification information.

Other technical changes implied by this commit
 * visibility is simplified to either public or invite;
 * can_see and can_see_full are removed, as well as concept of 'partial'
   display of slides;
 * group_ids related to private access is removed;
 * a computed / searchable field is added to be able to efficiently find
   and search active members of a channel;

This commit is linked to task ID 1937411 and PR #30847.