Make sure youtube playlists are being rendered properly

Right now it's possible to use 2 different urls for playlists which will work when accepting cookies.

• {{< youtube "videoseries?list=PLy7t4z5SYNaRYI7lH26szREOi9pfqxOUU" >}}
• {{< youtube "playlist?list=PLy7t4z5SYNaRYI7lH26szREOi9pfqxOUU" >}}

The thing with the first option is that is will not work if a user doesn't accept cookies. It will not create the thumbnail just like other videos.

The 2nd option on the other hand will create the thumbnail.

We should alter our Youtube plugin so that it accepts both urls but the code would replace "videoseries" by "playlist" if the string is present in the url.