Use ESLint, Prettier for code linting+formatting

[simon04]

• ESLint for JavaScript linting: https://eslint.org/
• Prettier for consistent JavaScript and CSS formatting: https://prettier.io/

npm run test finds issues w.r.t. ESLint and/or Prettier config

[perliedman]

Nice, I'm in favor of cleaning up the current mess in one big swoop (and then keep it clean with prettier) - but I guess we should coordinate when to merge this, so we don't accidentally make other valuable PRs invalid.

As it is now, this already has conflict, but I guess they should be easy resolve by rebasing.

[simon04]

I rebased onto master. Let's merge it right away, I'll take care of my single other open PR #189 since that might need some discussion.