Object Storage backend

In order to sustain growth of instances, not having to do mega-replication of GBytes of assets and help #64 (closed), having to ability to use a object storage for attachments can be really interesting.

• Use S3 api as an attachments storage
• Use Swift api as an attachments storage
• Use B2 api as an attachments storage
• Option to have a pleroma-managed cache of attachments, like use 10 GByte for lastest attachments locally to not use the upload bandwidth (not cheap in general) of thoses services.