preview html changes

Feature: preview Stage URLs in a new tab. 
Consists of: Small HTML/JS changes. 
Notes: 
* JS should be moved from HTML into separate file in static.
* There exists a check for "http://" because of undesired behavior where the lack thereof would result in Django directing a sample url "www.example.com" to "ab-testing-tool-server/www.example.com" instead of the desired "http://www.example.com"
I think we can be a bit more efficient and transparent here by defining a class-based click handler for the preview link in the javascript tag below instead of creating click handlers directly on each element.  I'd suggest giving the preview link a class and attaching a click handler to that class in javascript below.  You wouldn't need an id for the link since you can refer to `this` to change the href, and you'd have the option of retrieving the input value by id or by going through the parent.