[FIX] website_slides: set slide as completed only for valid members
[FIX] website_slides: set slide as completed only for valid members

Indeed some slides are available for external people if is_preview is set
to True. It indicates slide is available for teasing. It should not be
marked as completed for non members.

This commit is linked to task ID 1942836 and PR #31239.

Co-Authored-By: Aurélien Warnon <31f887109a40300a811b4c53b824bd37028b7744@odoo.com>
Co-Authored-By: Thibault Delavallée <2d325771bb6e8870d87f0f3f7b45589a8338e550@odoo.com>
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo