Rebase Tor Browser patches for Firefox ESR 68

We need to start rebasing our patches against Firefox 68. This is the ticket that tracks the whole effort.

It's helpful how we did it the last time: comment:6:ticket:25543. As mentioned there https://torpat.ch/ is a very valuable resource. It might need updating, though (which we should do while we are at it, or point Arthur to the things that need to get fixed).