Choose linting options

We're currently using https://github.com/azz/pretty-quick to run `Prettier` when building/deploying, but we don't actually have any rules defined in `.prettierrc`.

We should choose a common style guide and refrain from editing it. No one will be happy with everything, so better to choose one that everyone is slightly happy with.

We should also get pretty-quick to run ESLint, as `Prettier`'s configuration seems limited to things like white-space and punctuation. This may be enough but ESLint offers far more rules.