Consider removing/replacing babel-polyfill

[toolness]

This is partly because it's deprecated, as the following is logged whenever we build the site:

Creating an optimized production build...

  `@babel/polyfill` is deprecated. Please, use required parts of `core-js`
  and `regenerator-runtime/runtime` separately

However, I'm also not entirely sure it's necessary: react-app-polyfill/ie11, which was introduced in #99, might provide us with everything that's needed already.