[IMP] website: add a z-index on the header for shadow option

We need a z-index on the header to make the shadow option work properly
We also needed to increase the z-index of the popup snippet so that it
goes above the header when it is displayed.

Part of https://github.com/odoo/odoo/pull/49142


task-2229551

closes odoo/odoo#49142

Related: odoo/design-themes#238
Related: odoo/upgrade#1432
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>