rails/activestorage: Store files in Rails applications

A key difference to how Active Storage works compared to other attachment solutions in Rails is through the use of built-in Blob and Attachment models (backed by Active Record). This means existing application models do not need to be modified with additional columns to associate with files. Active Storage uses GlobalID to provide polymorphic associations via the join model of Attachment, which then connects to the actual Blob.These Blob models are intended to be immutable in spirit. One file, one blob. You can associate the same blob with multiple application models as well. And if you want to do transformations of a given Blob, the idea is that you'll simply create a new one, rather than attempt to mutate the existing (though of course you can delete that later if you don't need it).