Embedded backoffice: Add a query-param to render backoffice without navigation to be embedded in iframes

[ambroisemaupate]

• Add a Twig global scoped to /rz-admin path to check if iframe=1 query-param is set
• Change all backoffice template inheritance to use a simple base layout without navigation and trees

Check with @manuelodelain for additional frontend features

• Send a postMessage when saving items to broadcast a message to iframe ancestor