This fixes a few issues we've had for years.
First, when attaching an image and then sending a form with validation
errors, the image preview would not be rendered when the form was
displayed once again. Now it's rendered as expected.
Second, when attaching an image, removing it, and attaching a new
one, browsers were displaying the image preview of the first one. That's
because Paperclip generated the same URL from both files (as they both
had the same hash data and prefix). Browsers usually cache images and
render the cached image when getting the same URL.
Since now we're storing each image in a different Blob, the images have
different URLs and so the preview of the second one is correctly
displayed.
Finally, when users downloaded a document, they were getting files with
a very long hexadecimal hash as filename. Now they get the original
filename.
There could be inconsistencies in the database and an attachment might
have a `record_id` pointing to a record which no longer exist. We were
getting an exception in this case.
Just like we add the `storage_` prefix for new records so we can use
both Active Storage and Paperclip at the same time.
Now the migration actually works, at least for basic cases.
