Allow to upload images in development

[nashbridges]

This is part 2 for the migration.

It allows a developer to upload her/his own covers to s3. This way we don't have to have a special image processing proxy in development and can benefit from the same production proxy (currently imagekit.io).

IAM user is specified as write-only and is scoped to /dev folder, so that an attacker can't use them to overwrite/delete production images:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": [
                "s3:PutObject",
                "s3:DeleteObject",
                "s3:GetBucketLocation"
            ],
            "Resource": [
                "arn:aws:s3:::ua-books",
                "arn:aws:s3:::ua-books/dev/*"
            ]
        }
    ]
}

See markevans/dragonfly-s3_data_store#30 (comment) on why s3:GetBucketLocation is needed.