Better thumbnail quality

Current code to generate thumbnails does not produce good quality thumbs. See for example some of the crop images in the crops list page.

The issue is most visible with very high-quality source images, compare the generated thumbnail to the original image:

• https://beta.genesys-pgr.org/10.18730/H8F64
• https://beta.genesys-pgr.org/10.18730/H8PVV
• https://beta.genesys-pgr.org/10.18730/HTM8R