[FIX] website_slides: save video thumbnail on slide creation
[FIX] website_slides: save video thumbnail on slide creation

Linked to issue: https://github.com/odoo/odoo/issues/43745

At slide creation, as image_1920 was in readonly mode in form view when type is
video, document or presentation, the field was not sent in value to the create.

There is no reason to set this field readonly. If the user want to modify the
thumbnail, that his responsability.

This commit remove the readonly flag for slide.image_1920 field to allow
computed thumbnail to be saved at slide creation.

Task ID: 2182638
PR #44405

closes odoo/odoo#44472

X-original-commit: 15af8734c5d16a358cababb6163863b30e24f8cc
Signed-off-by: Yannick Tivisse (yti) <200a91eb0e5cc4726d6a3430713b580138f34298@odoo.com>

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