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 this check is fine for now, but we'll probably want to make the html5 input field of type `url` and actually carry through with the preview click if the value is valid.