「Ruby on Railsで動的に追加・削除可能な入力フォームの実装手順
https://www.virment.com/add_and_remove_rails_nested_form_dynamically/
gem 'nested_form_fields'
gem 'jquery-rails'
これが無いと動かない
//= require jquery <<
//= require rails-ujs
//= require activestorage
//= require turbolinks
//= require_tree .
//= require nested_form_fields <<
Uncaught ReferenceError: $ is not defined