Fix eslint warnings and format with prettier

On current master, eslint shows a dozen or so warnings when doing yarn start (or yarn lint). None of these are very critical, but they still don't look nice. By fixing all of them now, we can then more easily spot if a future MR has an actually relevant warning.

We also have a Prettier formatter config, but it hasn't been applied in quite a while. I've now done that. We should be thinking about doing that automatically in a pre-commit hook or something like that.