Adopt Bootstrap's "embed-responsive"

[nfreear]

Done! http://nick.freear.org.uk/2015/10/02/open-media-player-jisc-challenge.html

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• http://getbootstrap.com/components/#responsive-embed
• https://gist.github.com/nfreear/fe9e706b602960a76da3
• https://github.com/twbs/bootstrap/blob/master/dist/css/bootstrap.css#L5791-L5816