[Media size setting] Change thumbnail media fit to "fill"

**PR SUMMARY**
This ticket is a branch of media fit ticket. Issue: [2016](https://github.com/Shopify/dawn/issues/2016)

**Update on thumbnails:** The thumbnails will always be set to media fit "fill" on desktop and mobile - no matter the media fit option or asset type the merchant selects for desktop. 