Store images in highres/thumbnail formats

[baxgas]

Have two additional versions for every image:
thumbnail (square crop, 450x450px, jpg-medium), high (original aspect ration, max. 1600x1600px, jpg-high)
also the corresponding API endpoints: cover_image_url_thumb and cover_image_url_high
(actually 3 versions, since the original image could also stay)

maybe this? https://auth0.com/blog/image-processing-in-python-with-pillow/

[gammaw]

@baxgas @pvj I wasn't sure if we considered this for rework MVP? @molnar-a told me that the high res pictures are an issue for the app and it would be good to have smaller sizes. I now added this issue to https://github.com/mmmnmnm/lahmacun_arcsi/milestone/7 assuming that we need to do it for the MVP, but we can remove it if we decided that it's not needed.