Followup S3 blob work

[ktuite]

Followup to issue #585 after merging PR getodk/central-backend#1071

Must have:

• Add parameter to s3 CLI to upload specific number of blobs #703
• Log nightly run of s3 upload to audit log #704

To consider:

• Move pgrowlocks requirement to tests s3: remove PGROWLOCKS extension from db-migrations central-backend#1190
• logging events? upload succeed, upload failed etc.?
  make an audit log event from nightly processing job
• should migrating back from s3 to postgres be supported?
  • "Once you opt into using S3-compatible storage, you can't opt back out of it and migrate files back to the database." (central-install: document s3 config docs#1859)

For after .2:

• nginx config to mask URLs: s3: handle 307 redirects #716

Noting ideas/questions that came up but that we will park for now:

• tests for missing blobs, e.g. a single missing blob in a data export?
• are there race conditions in purge code? purge vs blobs re-attaching?