Add polyfill for older browsers to stop frontend erroring on use of flatMap

[darrenvong]

I was seeing a few c.data.pages.flatMap is not a function errors on Sentry and noticed that they are all from users from using older versions of Safari... this polyfill should hopefully fix it. It only gets applied to browsers who don't support the function already using the browserslist config

Closes #1823

Frontend checklist

• Formatted my code with yarn format && yarn lint --fix
• There are no warnings from yarn lint
• There are no console warnings when running the app
• Added any new components to storybook
• Added tests where relevant
• All tests pass
• Clicked around my changes running locally and it works
• Checked Desktop, Mobile and Tablet screen sizes

[lucaslcode]

I don't think this solves the issue without a modification to our package.json. npx browserslist ">0.2%, not dead, not op_mini all" gives safari 13.1 as the earliest version.

[darrenvong]

Good point. Changing it to >0.1% seems to cover ios_saf 9.3 which I think is different from Safari on MacOS, and considering the Sentry error came from ios Safari 10, I think that should do the trick. Maybe any browsers that are used by less than 0.1% of people is not worth supporting after? 🤷