Improve site performance issues related to fonts and images

[stefandesu]

https://pagespeed.web.dev/report?url=https%3A%2F%2Fcoli-conc.gbv.de%2F reports a performance score of 92 which definitely is good, but could be improved. In particular, it points out the following issues:

• Ensure text remains visible during webfont load
• Serve images in modern format
• Properly size images
• Serve static assets with an efficient cache policy
• Images without dimensions

Some of these should be easily fixed, but especially the image-related issues might involve some technical changes. Ideally, we'd have an easy workflow to throw in any kind of image file and have a script that optimizes it in all the ways we need.

In my opinion, this is low priority since the score is already very good, but it should be kept in mind.

[stefandesu]

For images, we could additionally add something like https://github.com/evanw/thumbhash to have tiny-sized previews that can be shown as placeholders before images are loaded.