Fix lint tasks and introduce prettier

[cjcenizal]

Fixes #480. Also fixes a bug introduced by #450. Turns out we haven't had a linter since then -- whoops!

This contains a commit addressing #448, but it's been reverted.

Thanks to @w33ble for all of his help with this.

[cjcenizal]

Note that I've disabled prettier by removing it from the eslint rules config:

  "rules": {
    "no-console": "error"
  }

As you can see by 4a9e6bc, these rules will result in an enormous diff, which will cause significant merge conflicts in the PRs we have open. I've reverted that commit until such a change will be disruptive.

[pugnascotia]

We ended up adding Prettier in #1529 (for TypeScript files only).