Enable fallback player, use videos.code.org subdomain

* Move videos to videos.code.org subdomain
* Use YouTube instead of YouTubeEducation
* Enable fallback player

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After Brent's review:

* Add video fallback player embed page going to allow for testing at the new IT page: http://brian.learn.code.org:9393/educate/it
* Use video fallback player for unplugged videos
* Convert unplugged videos to be seeded as real Video objects so they generate icons to use for the fallback player
* Remove video assets from the unplugged localization files
* Add new helper that generates a video player with fallback enabled
* Separate out video, dialog and utils code out of `application.js.erb` and `application_helper.rb` and into individual files