Add button group and full width button styles

[colinrotherham]

Description

This PR aligns with GOV.UK Frontend by including:

1. Switch to full width buttons on mobile
2. Add new "button group" styling

The styles for .nhsuk-button-group were initially copied from GOV.UK Frontend:

• govuk-frontend/src/govuk/objects/_button-group.scss

But have now been updated to match NHS App Frontend spacing:

• nhsapp-frontend/src/components/button/_index.scss

From NHS App Frontend Button component research:

Full width buttons

Testing on the NHS App has found that full width buttons are not a problem for users with smaller screens. But we will continue to test if:

• full width buttons are problematic for users with low digital skills
• users don’t actually see them as actionable things to click

Checklist

• [ ] Tested against our testing policy (Resolution, Browser & Accessibility)
• [ ] Follows our coding standards and style guide
• [ ] CHANGELOG entry