[FIX] website: correctly toggle the preview for the countdown snippet

The countdown snippet has an end action that can be configured to show a
message when the countdown reaches zero. A button in the editor toggles
a preview of this message. However, a bug currently makes the preview
disappear whenever the snippet's widget is restarted... which occurs by
simply hovering some other options.

To solve the problem, the preview visibility is now controlled by a
separate css class s_countdown_enable_preview overriding d-none. This
way, the preview visibility no longer interacts with the widget's logic
and is no longer affected by the widget restarting.

task-2638366

Part-of: odoo/odoo#76343