Fixed bug with nav bar in playground mode

This closes #57 

The file `router.js` is in charge of loading relevant content and performing relevant actions when a user navigates to a page within the site. The function `populateChapters()` was called when loading lesson content but not called when just going to the playground. Including this function call within the playground route resolved the issue.

I didn't feel the need to use the asynchronous syntax (`populateChapters().done(function(){...});`) since none of the playground content is dependent on the lessons. However, not including this in the asynchronous format may have consequences that I haven't considered.