Add S3/Minio Support for API

S3 Integration for attachments etc. would make it easier to integrate vikunja into existing setups. (Uploads could for example be handled like in hedgedoc, where files are uploaded to minio and then the public minio endpoint is used to link to the picture in markdown)

This is planned at some point, just not in the near future.

If you feel up to it and want it faster, I’d love a PR.