[FIX] website_slides: limit auto subscription

In order to avoid spam and unwanted notification this commit limits auto
subscription on channel when

 * creating a channel;
 * automatically telling a channel a new publication has been done;
 * creating a slide;

As being a follower of a slide channel means following its slides user should
choose manually to be follower of a channel, notably through the UI.

A new way of managing subscription will come with elearning in mind and
therefore auto subscription should be limited.

Commit linked to task ID 1933820 and PR #30830.

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