Automatically close sidebar/navigation when clicking a link

[chrisheninger]

Currently on mobile, when clicking a link from the nav, the sidebar remains open even though the page changes. We need to add something that will close the mobile sidebar/nav when navigating to a new page.

React-Router allows you to wrap everything in a RouteListener component that checks previous location with next location and closes the menu accordingly– I'm willing to bet that Gatsby has a similar way of handling this situation but we'll need to dig in and look it up.