When adding a URL in the `Insert from URL` state in the media modal, attempt to show a preview of the content. Drop the unused width and height fields.

This will probably be iterated upon.

Props helen, jtsternberg, wonderboymusic.
See #15490.


git-svn-id: https://develop.svn.wordpress.org/trunk@28581 602fd350-edb4-49c9-b593-d223f7449a82