gallery lightbox crop fixes

- Temporary fix to stop the gallery lightbox picking the auto-cropped 4:3 aspect images (this can be replaced when the new media service comes in)
- Fix for the caption info panel overlapping the lightbox image at desktop breakpoint

![image](https://s3.amazonaws.com/giphymedia/media/85hqFWqODN8Mo/200.gif)

// @jamesgorrie @theefer