Show playlist description to viewer of playlist

[violoncelloCH]

Describe the problem to be solved
I often times want to share multiple videos together accompanied by a short informative text. Therefore I create a playlist, set a title and add my videos. There is a description field when creating playlist, but sadly this description that description can only be seen by the owner of the playlist and not in the players view for the persons I share the video to.

Describe the solution you would like:
The description could be displayed together with the title/name of the playlist (see screenshot):

Describe alternatives you have considered
If making the description field visible publicly could be a privacy concern for whoever is already using that field to store sensitive information only for themselves, an additional field could be added for a public description.

[Poslovitch]

I will be able to work on it when #4489 is merged.