[Fairground 🎡] Titlepiece tweaks

[cemms1]

What does this change?

• Extracts the pillars definition into their own component with Storybook story
  • There's a "showBurgerMenu" prop to allow variation between the desktop designs and mobile designs
• Leaves TODO comments for inclusion of new burger menu
• Tweaks subnav styles and allows scrolling horizontally when items do not fit in the space
• Tweaks Titlepiece component styles

Why?

As part of the work the Fairground team are doing to revamp the homepage, this is part of the task to update the "Masthead", which includes the new navigation elements collectively called the "Titlepiece"

See https://trello.com/c/KMzghGe3/23-masthead-titlepiece