This adds support for Azure blob storage using Paperclip, similar to
what's supported for S3 and other backends. It makes use of the
`paperclip-azure` plugin gem. As with S3, this is not loaded by default;
it's only used if the proper environment variables are set.
I'm using my fork of paperclip-azure because it incorporates a few new
features to make it feature-equivalent to S3 for Mastodon's purposes.
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