thumbnails for external videos

Write a script to automatically download thumbnails for all embedded videos. The filename should be the youtube ID:

https://img.youtube.com/vi/ID/maxresdefault.jpg

Download the thumbnails
Create thumbnail-vid shortcode (Disclaimer message + clickable image that opens video at timestamp in separate tab)
Adapt dummy_for_hugo.md (replace youtube shortcode by thumbnail-vid)
Adapt build_md.py (assuming that the youtube-thumbnail shortcode will need two arguments: one including the timestamp and one only with the id or name of the image ID.jpg)
Adapt documentation (users should know about the difference between thumbnails and youtube embedding)