Add CSV download button to Studio video upload page

@nasthagiri @clrux @BenjiLee 

http://studio.gwprice.m.sandbox.edx.org/videos/vpip/vpip/vpip
From the name of the method, one would expect the response to be in JSON form - although it clarifies this in the docstring.
Keep in mind, there is a future story to display the encodings (with their links) in the UI (https://openedx.atlassian.net/browse/MA-150).  So at that time, the information returned by this method would be the same as what's returned by videos_handler.  Of course we can revisit the naming again at that time.