Add `<ButtonSet>` component

# Objective

Add a component for grouping buttons:

```tsx
<ButtonSet>
  <Button submit>Submit</Button>
  <Button>Cancel</Button>
</ButtonSet>
```

This displays the buttons in a horizontal stack (when there is room) and stacked vertically (stretched the full width) when space gets tight.

- [ ] Update `<FormFooter>` to use `<ButtonSet>`.

# Rationale

Buttons often need to be displayed in sets.