Use React code-splitting

[openjck]

We added code-splitting using a third-party library in #127. React now ships with code-splitting tools. They don't appear to be seriously interfering with the third-party library (yet) but we should move to React-based code-splitting soon to improve performance and ease maintainability, as well as reducing the risk of the two interfering badly.

https://reactjs.org/docs/code-splitting.html