Upgrade to Tor Browser based on Firefox 68.2

This ticket is about importing the security update immediately before our release, while #16356 (closed) is about porting Tails to Tor Browser 9.0 (which must be completed earlier).

Feature Branch: feature/16693-tor-browser-9.0+force-all-tests

Related issues

Blocks #16209
Blocked by #16356 (closed)

Original created by @intrigeri on 16693 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information