Update all react router (and associated) code samples

[hwillson]

PR meteor/todos#261 updates the react branch of todos to use react-router 4 (and adjusts the codebase to follow the react-router 4 way of thinking). Once that PR is merged, we'll need to update the react-router code samples in the Guide to match todos.

[filipenevola]

I'm closing this issue because it's too old.

We are going to make many updates to the guide in the next weeks and it's better to focus on issues with recent activity. If you think this issue is still relevant please open a new one.