Reduce security prefs into a few groups

We should reduce the number of preferences into levels of security rather than the myriad of individual behavior controls we have now.

Reducing the number of options can reduce the ability of users to fragment themselves into different anonymity sets through fingerprinting.

Our translators will hate us for a while, but there will be less words to translate total.

I want to get the set down to the following options:

• Don't record browsing history or website data (enables Private Browsing Mode)
• Disable browser plugins (such as Flash)
• Restrict third party cookies and other tracking data
• Change details that distinguish you from other Tor Browser users

Those last two might be better as per-site options.

I don't think we need much else. The rest of the options will remain buried in about:config.

The last two should also be removed once we get a better per-site Privacy UI (see #5273 (closed) and the UI mockup in https://www.torproject.org/projects/torbrowser/design/#identifier-linkability)