First pass at `wpview` logic for the `[embed]` shortcode. URLs on a their own line are parsed as well. The toolbar will appear with the "remove" button when the view is clicked. Edit has not been implemented yet.

Props avryl, wonderboymusic.
See #28195.



git-svn-id: https://develop.svn.wordpress.org/trunk@28358 602fd350-edb4-49c9-b593-d223f7449a82