Auto-start S3 server for dev

[NathanFlurry]

Motivation

We'll be depending on the uploads module more and more. We should provide a default S3 server for development & testing.

Implementation

• Use SeaweedFS (licensed Apache 2, since Minio is AGPL).
• Pass the credentials to S3_ENDPOINT, S3_DEFAULT_REGION, S3_BUCKET, S3_ACCESS_KEY_ID, S3_SECRET_ACCESS_KEY.
• Update uploads module to read these env vars appropriately.