Issue Summary

This is affected by the recent cardback modernization changes. When a user adds a youtube link to the card comments, it will no longer preview as an embedded video. It will only display as a link. 

Steps to Reproduce

1. Copy a Youtube URL
2. Open or create a card
3. Go to the Comments section
4. Paste the Youtube URL
5. Click to Save the comment

Expected Results

Youtube URL will turn into an embedded video and you can click to preview and play it inside the card

Actual Results

It will open a new tab. It will show what is selected in your Link Preferences: 
https://id.atlassian.com/manage-profile/link-preferences 

Workaround

1. Edit the comment
2. Click the link to Configure
3. Select the Embed option
4. Save the comment

What the Embed option looks like: