Moved cross-env to production dependencies, along with a few needed babel dependencies.

I'm not fully confident I've hit the mark on this one. It passes all the tests and seems to start up and work fine (I did a yarn install with production true then built the docker image from that), but I'm pretty unclear as to how babel plugins work. I'm not sure if there's some way to tell which are required in production and which are only required for the transpiling. Indeed, it's unclear to me if the majority of them are even used (just based on the fact that only a few are actually in the babel:plugins property.

Fixes: #283 (closed)