Persistent Storage feature: Tor Browser security level

This was, by far, the most requested feature in the 2020 user survey.
#18006 (closed) and 3.14.1 would have benefited from this.
On the other hand, our personas are probably fine with having JavaScript enabled by default. Most probably they actually need it.
Meanwhile, people do weird stuff like add .tor-browser/profile.default/prefs.js or .tor-browser/profile.default/user.js and got into trouble in 5.8.

Edited Feb 11, 2023 by sajolida