Deposit: Adapt backend configuration so we can lift other backends than filesystem

Currently a small part of the deposit model store the deposit archives received on the filesystem (mounted as nfs to be persisted in saam in the static infra).

Lifting [1], we could make it transparently use another kind of storage backend (minio ~ s3 like, azure blobstorage, ...).

That will ease the migration to the elastic infra.

[1] https://django-storages.readthedocs.io/en/latest/backends/amazon-S3.html