New video thumbnails

This PR includes updates to the new video thumbnails.  The big change is we're using images for the play arrows instead of CSS.  

Note that the PR applies the new styles to the article and related video thumbnails, but we'll revert those before we merge this as this ticket is just for style updates. 

@kflorence - there's some changes to VE code in here so please have a look when you have time.