expose `extensions.torbutton.confirm_newnym`

https://blog.torproject.org/comment/284706#comment-284706

... New Identity. It just deletes your session without warning ...

I actually did this myself twice. Once when I first used it: I guess I kind of expected a page action or menu (a la the old onion button). The second time I clicked it accidentally because of it's proximity to the hamburger menu.