[Fairground 🎡] Expanded nav alignment and styling

[frederickobrien]

Following on from #11572, this adjusts the alignment and styling of the new masthead expanded nav components to align (eh? eh?) with new designs.

The components are not reactive with this implementation (i.e. the expanded nav is constantly in the expanded state) but it looks as it should in that state. With a delicious new veggie burger all the pieces will be combined.

Some things to note:

• The search bar has been extracted into its own component to allow for it to appear in two different places across mobile-to-tablet and desktop-plus
• Colours have be routed through the palette.ts hub rather than being imported directly from Source

What this does not do:

• Interactive veggie burger behaviour

To be followed up with another PR in the chain.

Screenshots

Desktop

LeftCol

Wide