Improve placeholder thumbnails for eclipsefdn-video widget

If the user does not accept cookies, the eclipsefdn-video widget will display a link along with an image. However, the problem is that the size of the image differs from that of the video player. Previously, to solve this issue, we would either adjust the layout to accommodate both, or we would add custom CSS to make them match.

However, it would be much simpler to just have the placeholder be as close to the embed as possible in terms of size.

Another fix we can do is get rid of the black bars that appear on the top and bottom of the placeholder. It looks bad in my opinion.