changing http:// to // on intro video urls

Firefox 23 has started blocking mixed content (https://blog.mozilla.org/tanvi/2013/04/10/mixed-content-blocking-enabled-in-firefox-23/) so the urls should be changed to https. (or "//" as per @nedbat's comment)

Tested manually in firefox by changing http to https in inspector and it worked. I also tested to make sure that my changes work, but they do require db updates to make sure the videos have https in their records...

Laundry list of mentions: @ormsbee, @nedbat, @singingwolfboy