fixed 'new user modal wont close on mobile'

-Hid intro video from new-user modal on mobile, since mobile browsers
won't autoplay.
-With the video player not loaded, the hideNewUserModal() function was
failing when attempting to call.stopVideo() on the undefined object.
This was causing the function to stop before calling .hide() on the
new-user modal.  Added a check to only call .stopVideo() if the youtube
player object is != undefined