[REF] website_slides: perform some model code cleaning

This commit squashes several fixes / light improvements related to the new
slides / elearning homepage [1][2].

Containing notably

 * clean promote strategy field on channel model: less entries as they
   are now used only to display lessons in a given order on channels of
   documentation type;
 * remove promotional fields, not used anymore since partial access on channel
   allowing to have some kind of small preview of slides has been removed;
 * use standard image field (image, image_medium and image_small) and use
   image_resize_image from tools like all other odoo applications;
 * improve image resizing to have medium images better looking on frontend
   and use that nice image_large that broke my build yesterday;
 * add last publication date change on channel;
 * clean dead code;

[1] new homepage task: ID 1936153 and PR #30770
[1] main elearning task: ID 1902304 and PR #29876

Co-Authored-By: Aurélien Warnon <awa@odoo.com>
Co-Authored-By: Thibault Delavallée <tde@odoo.com>