Refactor instructions editor to follow controlled component pattern

Currently the instructions editor is an uncontrolled component, which is to say that intermediate in-progress editing state is stored only in the DOM.

React officially recommends using controlled components instead, which stores intermediate state in React (in our case Redux) state, and has the form value mirror the props given. This is probably the right approach here (but maybe it’s not! Further research may be called for).

Probably best not to sweat this too much until #1394 is done (or at least concurrently with that task).

まず instructions editor と、controlled components に関するリンク先の React ドキュメントを確認し、次に #1394 が作業にどのような影響を与えるかを確認します。中間的な編集状態が React または Redux の state に保存され、フォームの値がその props を反映するようになれば、リファクタリングは完了です。