The static files S3 bucket has been getting a lot larger recently,
between subreddit stylesheets being in there and the static file cleaner
being disabled due to a bug. This is causing the deploy process to take
upwards of 3 minutes just to determine that no files need to be uploaded
to the bucket.
As a short-term workaround, this changes the uploader to check each key
individually with an S3 HEAD request rather than listing the whole
bucket. This is slower than best case of listing the bucket, but is
significantly faster than the current condition (~25 second runtime
now).
