Lock down S3 buckets

### Context

It's possible to download objects from our S3 buckets if you the URL to the object, despite the permissions to list the contents of S3 buckets being locked down.

### Problem or idea

We should make it so that you need to have permissions to the data to download it, for legal reasons.

### Solution or next step

However will want people to be able to pull data from those buckets eventually, a prime example of this is transcriptome indices. I think this is the strategy @Miserlou mentioned for enabling that: https://docs.aws.amazon.com/STS/latest/APIReference/Welcome.html

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