Add video playback support

Add video playback support

 - The StaticFileHandler gives us automatic support for mime type and partial file download using the Range request header.
 - Subclass static handler to support video playback and also fo download
 - js changes to preload video and then show video node if video loads
 - progress indicator while video downloading
 - error indicator when video failed to load with error message
 - do not show video on older devices
 - hide video if there is a problem
 - reset width and height properly on every preview load