Prevent users with no script right to add Gadgets with velocity code or add a warning when they try to add one

WIth the change done on ticket XWIKI-20321 at the moment when a user with no script rights adds a new Gadget to a Dashboard, ex. the default one, he breaks all the other gadgets that contain velocity code.

The user shoul aither be informed before adding a gadget that this might happen or to not have the option to add the gadget unless he has the required rights.