Tweaked some of the default Next.js rules, fixed lint-staged for `next lint`, made a few small easy lint fixes. Feels good!
Note that using the `dirs` option in `next.config.js` was causing `lint-staged` to lint _everything_. That's why I edited `yarn lint` to specify the dirs instead: that way, that command will lint all those dirs, but they won't get included in invocations with `--file`.
There are still a few lint errors left after this commit, because our <img> tags aren't working (@next/next/no-img-element). I'll fix those when we figure out what's wrong with images!
