[IMP] website: add the ability to create mega menus

Allow users to create complex website menus, also known as "mega menus".
Mega menus are enabled individually via the edit menu dialog, then their
content can be customized by dragging snippets in them like any other
snippets area in edit mode.

task-1925319

closes odoo/odoo#36097

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


Co-authored-by: Andrea Ulliana <aul@odoo.com>