Improve video thumbnail images

[jonoaldersonwp]

Video thumbnail images (those in 'grids layouts') on pages like ​https://wordpress.tv/, ​https://wordpress.tv/category/year/2017/ and ​https://wordpress.tv/tag/translation/page/2/ (excluding the video/thumbnail in the hero 'player' element when present) should be loaded more efficiently.

Specifically:

• The thumbnail should be generated/requested at an appropriate size (160x100)*
• A loading="lazy" property should be added.
• Explicit height and width properties should be added.
• If available, an srcset property should reference a x2 size option (at 320x200).

*E.g., ​https://videos.files.wordpress.com/bjn9Fo1g/video-33873a5e9c_mp4.scrubthumb.jpg, which is currently on the homepage, is loaded at 1920x1080, but squashed into a 160x100 space