Video editing in the media modal:

* Add a state: `Add Subititles`
* Add `text/vtt` to the list of allowed mime-types, files end in `.vtt`. `.srt` files are served as `text/plain`.
* The content body of a video shortcode should be used for adding `<track>` elements only. This happens dynamically in the modal. If added by hand, they can still be parsed and managed.

See #27016.



git-svn-id: https://develop.svn.wordpress.org/trunk@27481 602fd350-edb4-49c9-b593-d223f7449a82