ESLint non-client code

[peterbe]

Since we rely on create-react-app for the client package, it comes with a robust and large-community maintained eslint configuration.
For the record, it's tested automatically when you run cd client && yarn run build but it's also in effect when you run cd client && yarn run start (aka. make run-dev).

But we have lots of other .js files outside of client that could benefit from eslint validation.