Get current Tor Browser code ready for RLBox

RLBox is written in C++17. We likely hit build issues in an ESR 68 environment with a C++17 requirement. There is the option to adapt RLBox to C++11 code but we should not follow that path but backport build related patches instead.

https://bugzilla.mozilla.org/show_bug.cgi?id=1560664 is a ticket to start.

If that turns out to be too tricky to require C++17 for the whole codebase we can think about enabling it just for the part we want to start sandboxing.