Lesson scroll informational alert notification

When the navigation bar is hidden as a user scrolls down a lesson page, a Bootstrap information alert is shown. This notification details how to once again make the navigation bar visible. When a user clicks the 'x' icon in the alert, a key is added to the user's localStorage. This key is used to prevent the alert notification from being shown each time a lesson page is rendered. As the server does not need this information, localStorage allows functionality similar to cookies without any additional HTTP requests. 

Manually tested in IE8, Firefox 5, Chome 36, and Safari 7. localStorage is supported in IE8+, Firefox 3.5+, Safari 3.1+, and Chrome 4+.

Demo: http://mysterious-fortress-9395.herokuapp.com/introduction-to-web-development/tools-of-the-trade

To view/clear your browser's web storage, use ``` localStorage ``` / ```localStorage.clear()``` in the console.