Use Active Storage to handle cached attachments

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.

lib/tasks/files.rake

@@ -1,18 +1,8 @@
 namespace :files do
   desc "Removes cached attachments which weren't deleted for some reason"
   task remove_old_cached_attachments: :environment do
-    paths = Dir.glob(Rails.root.join("public/system/*/cached_attachments/**/*"))
-    logger = ApplicationLogger.new
-
-    paths.each do |path|
-      if File.file?(path)
-        if File.mtime(path) < 1.day.ago
-          File.delete(path)
-          logger.info "The file #{path} has been removed"
-        end
-      else
-        Dir.delete(path) if Dir.empty?(path)
-      end
-    end
+    ActiveStorage::Blob.unattached
+                       .where("active_storage_blobs.created_at <= ?", 1.day.ago)
+                       .find_each(&:purge_later)
   end
 end