fix fileURL to return absolut url to cdn

fileURL should contain a fully resolvable URL to the CDN service including bucket. As these are config variables, they are not part of the path stored in the meme repository.

Needs a forgotten ENV MINIO_CDN_URL=https://memesearch-cdn.lehre.ris.bht-berlin.de, therefore blocked by !9 (closed), so it can be added to the merge request.