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).

先查看 instructions editor 以及链接的 React controlled components 文档，然后检查 #1394 会如何影响这项工作。当中间编辑状态存储在 React 或 Redux state 中，并且表单值与其 props 保持一致时，重构就完成了。