Add thumbnail imange for video blocks in Ontraport Mail

When using an Ontraport hosted video it would helpful, by saving time and clicks, for a thumbnail of the video to appear in emails. Yes, the video itself won't play, but if the file is hosted by Ontraport it should be possible for the system to generate a thumbnail based on that file, which can be linked to the actual video in a separate destination.

- On behalf of Dr. Dalfino