Support S3-compatible blob storage

[alxndrsn]

Closes getodk/central#585

Adds:

• s3_status column on blobs table
• CLI tool for:
  • uploading blobs to s3
  • resetting failed blobs -> pending

Changes:

• blob.content uses must now be accompanied by a conditional fetch of the data from s3

Queries:

• would it be preferable to make blob.content async everywhere, and moving the s3 check/fetch inside the getter?
  • pro: neater code, less likely to have bugs where blob.content used when data is not available
  • con: seems intrusive of frames
• should "purging" forms in central also delete content from S3? yes