[FIX] website_slides: various models improvements

Purpose of this commit is to fix and improve some model related bits of
code:

 * add an ondelete cascade on some required fields;
 * make a public method private because it should be accessed only through
   controllers;
 * do not add odoobot as channel member; notably for demo data it can create
   unnecessary of noise in members and notifications;

Commit linked to task ID 1941250 and PR #31453

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