browser: alternative media player: the new `alt_media_player` module implements an `ogv.js` based media player which is able to play WebM (VP8/VP9) or Ogg (Theora, Vorbis) formats (+ other codecs not integrated yet) on browser that don't do it natively. The module imports `ogv.js` only when it's necessary (incompatible browser + videos needing it). This for now used in blog, and will be used in other places when it's suitable.