BUG: Youtube video embed should disappear after removing its URL from comment

[adrienj]

Steps to reproduce:

1. Open any Trello card of yours

2. Paste a Youtube URL (e.g. `https://www.youtube.com/watch?v=mSgza7elZt8`) as a comment of that card, and save the comment => the Youtube video is embedded below the comment

3. Edit the comment again to remove the URL, and save => the Youtube video embed should disappear but it does not.

[adrienj]

FYI, I just submitted this bug through https://trello.com/contact, as suggested by Trello on Twitter.