Add lint rule to ensure `<img>` is self closing

When <img> is does not have self closing slash like <img />, vercel deploys broke completely.

<img > is considered valid HTML as it is a void element, but for some reason <img > breaks vercel deploys.

Apparently <img> is not valid in XHTML or XML and needs to be <img /> so maybe XHTML or XML is being used under the hood somewhere? Not sure

We should investigate why this breaks and/or add a lint rule so we catch this at the linting stage and prevent future headaches.