🎉 Support ActiveAdmin 2.x and code style improvements

- Add support ActiveAdmin for 2.x
- Add rubocop config
- Add frozen_string_literal to ruby files
- Apply changes to respect rubocop alerts

lib/activeadmin/quill_editor.rb

@@ -1 +1,3 @@
+# frozen_string_literal: true
+
 require 'activeadmin/quill_editor/engine'