* Videos should always render at the same aspect ratio.

* Don't force a pseudo-mime-type for `.m4v` files
* Uniformly adapt to `$content_width` when setting video dimensions on the front end
* Add the `height` attribute to the initial `<video>` in the video playlist JS template
* Add some defensive/responsive CSS for a/v on the Edit Media page

See #27243.


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git-svn-id: https://core.svn.wordpress.org/trunk@27180 1a063a9b-81f0-0310-95a4-ce76da25c4cd