This adds a new check to the playlist url_id check to allow playlist urls from subscription only content on youtube channels.
The channel I tested on had an ID length of 26 (https://www.youtube.com/playlist?list=UUMOvk0KB4Ue0vfPqvDzjIAwiQ), I am not sure if other lengths are valid here also.
I have tested this on my local install and it is downloading the members only playlist correctly as long as I have valid youtube cookies.