Add a timestamp to the urls passed to `<audio>` and `<video>` in the modal to ensure that cached view instances aren't referenced by MEjs. Pause the player when closing the controller's modal.

See #27016, #26779.



git-svn-id: https://develop.svn.wordpress.org/trunk@27420 602fd350-edb4-49c9-b593-d223f7449a82