[FIX] website: restore scroll effects in custom code

The website scroll effects using the `AnimationEffect` Odoo API were not
working anymore if custom code enforces the html element as the
scrolling element. Indeed in that case the scroll event handlers were
now attached to the html element itself while it needs to be the window
in that case.

This may be something to fix at the source (the `getScrollingElement`
method). This will be checked later.

closes odoo/odoo#69083

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>