Backports needed for Firefox/Thunderbird ESR 68 in Stretch

[Moritz Mühlenhoff <jmm@inutil.org>]

Hi,
Firefox 68 will be the next ESR release series. With the release of Firefox 68.2
on October 22nd, support for ESR 60 will cease.

ESR 68 will require an updated Rust/Cargo toolchain and build dependencies not
present in Stretch (nodejs 8, llvm-toolchain-7, cbindgen and maybe more).
Stretch was already updated wrt Rust/Cargo for ESR 60, so there's at least no
requirement to bootstrap stage0 builds this time.

If we want to continue to have Firefox/Thunderbird supported in oldstable-security
after October, someone needs to step up to take care of backports to a Stretch point
release before October 22nd (or in case of poor timing, we can also release build
dependency updates via stretch-security).

Cheers,
        Moritz