Maximum image resolution

Similar to the beta feature for the [image widget file size limit](https://github.com/netlify/netlify-cms/blob/master/website/content/docs/beta-features.md#image-widget-file-size-limit), I'd like to propose a maximum image resolution setting. There are cases where the maximum resolution is important, for instance when using [gatsby-plugin-sharp](https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-plugin-sharp) the build process runs out of memory during conversion to WEBP/AVIF if the image has too many pixels. A large resolution image with a low quality setting can get through a simple file size filter.