[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

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>

addons/website_slides/views/slide_slide_views.xml

@@ -46,8 +46,7 @@
                         </div>
                         <widget name="web_ribbon" title="Archived" bg_color="bg-danger" attrs="{'invisible': [('active', '=', True)]}"/>
                         <field name="image_1920" widget="image" class="oe_avatar" options='{"preview_image": "image_256"}'
-                            attrs="{'readonly': [('slide_type', 'in', ('document', 'presentation', 'video'))], 'invisible': [('is_category', '=', True)]}"
-                        />
+                            attrs="{'invisible': [('is_category', '=', True)]}"/>
                         <div class="oe_title">
                             <div>
                                 <label for="name" string="Content Title" class="oe_edit_only"/>