Generate previews for large images so they load faster

[haku]

currently all images are served as is when viewed in the /i/ item page. photos can easily be 10 MiB or more, making them very slow to load.

similar to generating thumbnails, also generate medium sized images for faster viewing. to decide:

• file size threshold? maybe 1 MiB?
• what options to resize to? maybe jpeg 80%, capping to 2000px in both dimensions?
• reuse --thumbs directory or use separate directory? separate is more config but doubles as a feature flag which is nice.