Make uploads behavior parametrizable via env variables

- UPLOADS_LOCATION: "local" means use a local uploads folder, "s3" means use an AWS S3 bucket. - new secret to configure AWS region to use, in case S3 is used for uploads. - if S3 is used for uploads, AWS_S3_BUCKET env variable sets the bucket name ("feedbunch-<environment>" by default, e.g. "feedbunch-production" for the production environment.