Upgrade to NHS Frontend v9

[edwardhorsford]

NHS Frontend v9 was released last week. There are a few breaking changes in it.

We should first do fixes for #240 and then for this one.

[edwardhorsford]

We should also read through v8.3 and see if any updates are needed for that.

[rowellx68]

Hi, I maintain a separate React implementation here that supports v9. It has a different API to this implementation though, but if there's an appetite for the changes I've made I would be happy to create a PR here.

Example differences

<Button
-   secondary
+   variant="secondary"
>Discard changes</Button>

-<ButtonLink href="/page/2">
+<Button as="a" href="/page/2">
  Next page
-</ButtonLink>
+</Button>

-<TextInput inputRef={inputRef} label="Some label" />
+<Input ref={inputRef} label="Some label" />