redirect all traffic to / so React Router can handle the request

e.g. visiting https://mergefest-oxnulsy-ulyecw4ca3wk6.eu-2.platformsh.site/rabbithole doesn't work since nginx tries to respond to it. All frontend URLs should be handled by the same handler, then the router kicks in.

And please, use reach router instead of react router (because https://reacttraining.com/blog/reach-react-router-future/)
 