Refactoring using proper react-router practices

[Lucretius]

Small bit of refactoring:

1. When I originally set up the routing, I was unaware of the proper method for nested routes in react-router. Having figured them out recently, I figure it would be a good time to correct my mistakes here. Basically, the root route serves an App component, which includes the header and side menu I want on all child routes. The this.props.children line in App renders whatever child route component visited (Secrets, Settings etc.)

2. Not sure why I implemented a polling mechanism for checking logout when I'm given the exact time. I've replaced the setInterval calls with a setTimeout in componentDidMount which warns exactly two minutes prior to expiration and another setTimeout to boot the user back to the login page. I do not bother to clear the timeout on unmount, because the only time the App component will unmount is when logout occurs.

[djenriquez]

LGTM, much cleaner, nice work.