Allow object storage to work with S3-compatible storage solutions

[arkirchner]

ActiveStorge blobs can currently only be saved in a local storage (on disk). We want to support S3-comparable storage to support containerization.

Steps:

• Add optional configuration to use S3 as storage
• Mirror local blobs to S3 storage
• Add remaining local data to S3
• Use S3 as default provider
• Remove mirroring and only use S3
• Remove local data