Improve a video-link thumbnail

For school administrators using GiveCampus Outreach. Controls can differ between the classic email editor and the newer Composer editor.

The video-link insertion chooses the video's thumbnail-with-play-button when available, otherwise its thumbnail, and inserts a linked image. Update/select the intended video thumbnail where that interface permits and reinsert/review it, or upload an approved image and link it to the video URL using ordinary image/link controls. Preserve the destination and test the delivered email. The thumbnail remains a click-to-watch link, not an embedded in-email player.