Add left/center/right alignment to block advanced settings

This is a question that has come up so many times. While bootstrap does provide CSS classes to do this, editors do not usually understand CSS and are confused on how to change alignment.

Therefore I am proposing we add a radio button to advanced settings to toggle the alignment. It would simply add a CSS class to the respective element. Default should be "default" which would not apply any class.

Would appreciate feedback from others about this issue. Once again, the line between content and design is really hard to define, we're mostly playing it by ear based on feedback we get from developers and editors on what things frustrate them or would make their life easier.

まず、ブロックの advanced-settings エントリーポイントと、既存の Bootstrap クラス処理を見つけます。左/中央/右のラジオコントロールがデフォルト状態をどのように表現し、それぞれの要素に配置を適用すべきかを確認します。エディターが各配置を選択でき、デフォルトのオプションではクラスが追加されなければ完了です。

索引モデルが issue の本文から書いたものです。