title: Wagtail video embed fails for some YouTube URLs but not others
body: <p>I have a Wagtail 2.3 instance, and am embedding videos in templates from a variety of sources. A small portion of YouTube video URLs fail when attempting to embed, despite appearing to allow embedding. If I call <code>get_embed</code> directly, I see a HTTP 401 Unauthorized error and <code>get_embed</code> returns None. If I use the video embed widget on the page editor, for these URLs it fails with "Cannot find an embed for this URL."</p>
  
  <p>Manually embedding any of these videos with an iframe from the "Share" frame options on these videos works fine. </p>
  
  <p>An example of a video that is failing: <a href="https://youtu.be/rCvKr6nsS3w" rel="nofollow noreferrer">https://youtu.be/rCvKr6nsS3w</a></p>
  
  <p>Has anyone experienced similar with Wagtail video embedding or oembed generally? Does anyone know why these particular videos would be failing?</p>