Add StrictMode to React component root

[outoftime]

React 16.3 introduces <StrictMode>, which provides extra runtime warnings for deprecated or potentially buggy behavior. We should add it to the root component.

[alexpelan]

blah. so every one of our StrictMode errors is currently caused by react-redux and our connected components. This is fixed in react-redux@next (see: reduxjs/react-redux#897 ). Unfortunately, Popcode doesn't seem to boot with this version of react-redux (see: reduxjs/react-redux#965 and others).

The good news is I think we can just wait for the next cut of react-redux, yarn add that, and we'll be good then.