Swap S3 to first-party client

Summary:
Ref T5155. Swaps Phabricator over to the new first-party S3 client using the v4 authentication API so it works in all regions.

The API requires an explicit region, so the new amazon-s3.region is now required. I'll write guidance about this.

Test Plan:

• Uploaded files to S3.
• Migrated ~1GB of files to S3.
• Loaded a bunch of files off S3.
• Browsed around the S3 bucket.
• Deleted a file, verified the data on S3 was destroyed.
• Hit new setup warning.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5155

Differential Revision: https://secure.phabricator.com/D14982