Fix #4260: Unable to replace cdn.donmai.us images?

Bug: Replacing posts hosted on cdn.donmai.us didn't work.

Cause: Original files on cdn.donmai.us are hosted under /var/www/danbooru/original/, but replacements
were trying to store them directly under /var/www/danbooru, which failed with a permission error.
We were trying to store them in the wrong directory because we didn't respect the `original_subdir`
option when generating file paths.

app/logical/storage_manager.rb

@@ -85,7 +85,7 @@ class StorageManager
     when :large
       "#{base_dir}/sample/#{subdir}#{file}"
     when :original
-      "#{base_dir}/#{subdir}#{file}"
+      "#{base_dir}/#{original_subdir}#{subdir}#{file}"
     end
   end
 

test/unit/storage_manager_test.rb

@@ -130,6 +130,18 @@ class StorageManagerTest < ActiveSupport::TestCase
         assert_equal("http://localhost/images/download-preview.png", @storage_manager.file_url(@post, :preview))
       end
     end
+
+    context "when the original_subdir option is used" do
+      should "store original files at the correct path" do
+        @post = FactoryBot.create(:post, file_ext: "png")
+        @storage_manager = StorageManager::Local.new(base_dir: BASE_DIR, base_url: "/data", original_subdir: "original/")
+
+        assert_equal("#{BASE_DIR}/original/#{@post.md5}.png", @storage_manager.file_path(@post, @post.file_ext, :original))
+
+        @storage_manager.store_file(StringIO.new("data"), @post, :original)
+        assert_equal(true, File.exist?("#{BASE_DIR}/original/#{@post.md5}.png"))
+      end
+    end
   end
 
   context "StorageManager::Hybrid" do