Cache generated webalmanak thumbnails

[Jelmer van der Linde]

The thumbnails in the webalmanak are now always generated on the fly. These should at least pass on the proper cache headers, and to reduce server load and load times, we might want to statically cache them server side as well.

[Jelmer van der Linde]

At least I added image sizes and proper caching headers to the generated thumbnails in b7ce1e9. Still, server side caching would be quite welcome.

[Martijn Luinstra]

Is this still an issue?

[Jelmer van der Linde]

I think I already solved this accidentally while implementing the circle-shaped and text avatars 😁