This will be used to streamline a common pattern in our code for
reacting to changes in state selected from the central Redux store.

The utility is a standalone function rather than a method of the store
so that it can easily be used with mock stores and also other data
sources in future.

```
watch(
  store.subscribe,
  () => store.getState().someValue,
  (currentValue, prevValue) => { /* Handle change */ },
);
```