Use minio as datastore instead of postgres

[matthiasschaub]

Since the Sketch Map Tool handles a lot of big binary data such as Images and PDFs Minio might be a better fit then Postgres as datastore. Minio is also supported by Celery. The downside would be that we no longer can store metadata in the same place as files (E.g. At the moment, uploaded files are stored with their metadata in one Postgres table). Keeping Postgres for Metadata is maybe a bit too much.