I moved `config/babel.js` to `.babelrc.js` because it seemed like babel
7 really wanted that. I also blew away our (complicated, copied from
create-react-app) config and replaced it with a much, much simpler one.
Test plan: `yarn test` passes, `yarn start` still serves a working
server, and `scripts/build_static_site.sh` still produces a working
site.
Possibly we lost some nice features re: React debugging; if so I'll add
them back as I miss them.
Dandelion Mané