Draft: Bug 40641: Add projects/firefox-l10n

Part of #40641 (closed).

This requires rbm!42 (merged). (for testing I did a git-checkout of this in ~/rbm)

With this patch I can now run:

~/rbm/rbm build firefox-l10n --target alpha --target torbrowser-linux-x86_64

Which creates the directory out/firefox-l10n/firefox-l10n-9098c1823e51-1/, containing one tar.xz file for each locale.