Use function components to reflect changes in CRA

[JacksonKearl]

Closes #8.

Also, this could use useContext, which would replace

<ResponsiveContext.Consumer>
   {size => 
     ....

with

const size = useContext(ResponsiveContext)

But that would require the side bar component to be extracted out to its own function, as the hook wont work outside of the Grommet call.