* 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.



git-svn-id: https://develop.svn.wordpress.org/trunk@27328 602fd350-edb4-49c9-b593-d223f7449a82