Use the storage_ prefix for migrated attachments

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.

spec/lib/tasks/active_storage_spec.rb

@@ -0,0 +1,40 @@
+require "rails_helper"
+
+describe "active storage tasks" do
+  describe "migrate_from_paperclip" do
+    let(:run_rake_task) do
+      Rake::Task["active_storage:migrate_from_paperclip"].reenable
+      Rake.application.invoke_task("active_storage:migrate_from_paperclip")
+    end
+
+    let(:storage_root) { ActiveStorage::Blob.service.root }
+    before { FileUtils.rm_rf storage_root }
+
+    it "migrates records and attachments" do
+      document = create(:document,
+                        attachment: nil,
+                        paperclip_attachment: File.new("spec/fixtures/files/clippy.pdf"))
+
+      expect(ActiveStorage::Attachment.count).to eq 0
+      expect(ActiveStorage::Blob.count).to eq 0
+      expect(test_storage_file_paths.count).to eq 0
+
+      run_rake_task
+      document.reload
+
+      expect(ActiveStorage::Attachment.count).to eq 1
+      expect(ActiveStorage::Blob.count).to eq 1
+      expect(document.storage_attachment.filename).to eq "clippy.pdf"
+      expect(test_storage_file_paths.count).to eq 1
+      expect(storage_file_path(document)).to eq test_storage_file_paths.first
+    end
+
+    def test_storage_file_paths
+      Dir.glob("#{storage_root}/**/*").select { |file_or_folder| File.file?(file_or_folder) }
+    end
+
+    def storage_file_path(record)
+      ActiveStorage::Blob.service.path_for(record.storage_attachment.blob.key)
+    end
+  end
+end