Update data syncing to delete missing images

[mihow]

Update the data sync method to delete source images in the DB if they are missing from the source. What are some ways we should handle this? Warnings to the user? Check if source images have already been processes and have detections, verifications. Do we soft delete or loose it for real?

Consider switching to s5cmd instead of boto3 to increase speed of sync? - OR first try to tune concurrency settings: https://boto3.amazonaws.com/v1/documentation/api/latest/guide/s3.html