[FIX] website: restore parallax previews in edit mode

When the user changed the parallax option, it was only updated on save.
So, the user was not able to have a preview of what he chose.

Note: this commit is made in saas-12.3 but does not fully solve the
problem there... and unfortunately I don't know why. It fully solves
the problem in 13.0 though and this is more important.

closes odoo/odoo#39991

X-original-commit: 4b633949cd4fd1cae807b95d233879ac4adc01ce
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>