Recognize YouTube links for IFrames and change to an embed link

[rowanc1]

A number of times people just copy the youtube link in, rather than going to the embed flow in the YouTube UI. These links are deterministic based on the URL, and so we can do that step for people.

Go from:

• https://www.youtube.com/watch?v=CtqjD1X_5QQ to
• https://www.youtube-nocookie.com/embed/CtqjD1X_5QQ

The transform should respect the start-at query param:

• ?start=42

This is for the iframe element only, and could look similar to the link transformers, or expand that pattern to look at iframes as well?