Add world map

[cjrace]

Overview of changes

Aim here is to have a world map coloured in for the countries that we've both been to.

Checklist

• I have tested these changes locally using pnpm test
• I have updated the documentation (if needed)
• I have added or updated tests for these changes (if applicable)

Reviewer instructions

Still a work in progress...

• Base map needs a bit of tidying
• Want a nicer way to store the countries (and need to make Laura's list accurate)
• Need to move to mantine components for the dropdown
• Need to plan how countries will look if both have been there