Re-enable Tor Browser's "Quantum bar"

To workaround #17121 (closed) (aka. https://trac.torproject.org/projects/tor/ticket/32019) we had to disable Tor Browser’s “Quantum bar”: dfeaa6e96e767dcd96304cceeb5aaa39ef2b4469.

Sooner or later, Mozilla will remove support for the obsolete thing we’ve reverted to, so we need a real fix at some point.

Feature Branch: bugfix/17143-17214-12023-remove-obsolete-tor-browser-customization+force-all-tests

Related issues

• Related to #17121 (closed)
• Blocks #16209
• Blocks #17214 (closed)
• Blocks #12023 (closed)

Original created by @intrigeri on 17143 (Redmine)