This commit adds an `enable_s3_transfer_acceleration` site setting,
which is hidden to begin with. We are adding this because in certain
regions, using https://aws.amazon.com/s3/transfer-acceleration/ can
drastically speed up uploads, sometimes as much as 70% in certain
regions depending on the target bucket region. This is important for
us because we have direct S3 multipart uploads enabled everywhere
on our hosting.
To start, we only want this on the uploads bucket, not the backup one.
Also, this will accelerate both uploads **and** downloads, depending
on whether a presigned URL is used for downloading. This is the case
when secure uploads is enabled, not anywhere else at this time. To
enable the S3 acceleration on downloads more generally would be a
more in-depth change, since we currently store S3 Upload record URLs
like this:
```
url: "//test.s3.dualstack.us-east-2.amazonaws.com/original/2X/6/123456.png"
```
For acceleration, `s3.dualstack` would need to be changed to `s3-accelerate.dualstack`
here.
Note that for this to have any effect, Transfer Acceleration must be enabled
on the S3 bucket used for uploads per https://docs.aws.amazon.com/AmazonS3/latest/userguide/transfer-acceleration-examples.html.
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