Create preference for DOM storage isolation and image cache isolation
In #6564 (closed), we created a patch to isolate DOM storage to first party domain. It could use a pref to control if it is enabled, and ideally also have an option to control if it only applies to private browsing mode windows (though that could be a separate ticket if it is substantial).
Our patch for the isolation is here: https://gitweb.torproject.org/tor-browser.git/commitdiff/1b3c110a29ae11b50ce2bf56d5954773262e67c0