Fix #2337: Added a warning to prevent accidently deleting a page

This should run through the `reporter` object. It's not loaded into ceci-card right now, but if you look at https://github.com/mozilla-appmaker/appmaker/blob/develop/public/designer/components/user-state.html#L907 it's relatively easy to add that to this custom element. And example of it being used for a confirmation: https://github.com/mozilla-appmaker/appmaker/blob/develop/public/designer/components/user-state.html#L245