Move privacy checkboxes to about:preferences#privacy (proposed)

I would like to propose moving the Privacy Settings checkboxes to the about:preferences#privacy page. That means removing them from torbutton and adding them to tor-browser.git

Here is my reasoning:

1. The security slider can be separated from the privacy settings. The privacy checkboxes are dangerous, because the default (all checked) offers maximum privacy, and unchecking any of the checkboxes substantially reduces privacy. Whereas the security slider has low security by default, and moving the slider always increases security. So I think it's better to encourage use of the security slider and discourage use of the privacy checkboxes.

2. We unify the privacy user interface, which hopefully simplifies it for users. There are already privacy settings for Firefox on the about:preferences#privacy page, so it seems natural to move them there. Also the "Always Use Private Browsing Mode" checkbox on that page is more or less equivalent to torbutton's "Don't record browsing history" checkbox. And then we don't have to duplicate the "restart" prompt associated with that checkbox.

3. Hopefully we can uplift to mainline Firefox, one of our SponsorU goals, thereby exposing our great privacy features (first party isolation, fingerprinting resistance, etc.) to Firefox users in the privacy UI.