[IMP] http_routing, portal, website: nicer 404 layout

This commit introduces a nicer 404 page, which is basically the same layout as
the one used on Odoo.com.

Also, the 404 is now fully customizable, blocks can be drag'd & drop'd.

task-1966460

closes odoo/odoo#38901

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>