Youtube privacy

[imorland]

Modifies the Youtube embed template to render youtube-nocookie.com in place of youtube.com, for privacy focussed reasons.

However, we need to consider:

• Forcing this may or may not be popular
• To date, this extension has been config free, so placing this behind an option would go against that
• Maybe this should be left to forum admins to set in their local extend.php
• Or this should be a seperate micro-extension.

Anyhow, I'll leave this here for a while to gather comments, opinions, etc