Add play controls to the challenge submission screencast

[nickserv]

This screencast is currently on the challenges page:

I think the content of it is great, but I'm concerned that it may be too long for someone to follow without play controls. I was curious about the length of it, and it may just be my browser, but I timed the screencast at 3 minutes and 56 seconds (until it loops). It would be great if we could either add play controls to this GIF somehow, or convert it to an HTML5 video or a YouTube video so that the user can pause and scrub the video. Also, longer animated gifs often aren't very efficient in reducing file size, so we could reduce our file size.

Another thing to note is that the screencast is using the challenge guide in Welcome, which is now deprecated. I don't think this is a big deal since it's only a short section of the screencast and it's fairly easy to find the challenges on the new site, but this might be a reason to rerecord the screencast if someone wants to do that.