[Feature Request] "Subscribing" to playlists

[Gthbacc]

Playlists that are already made can be updated by their creators. While the current import feature is actually really cool and useful, specialy for converting whole channels into playlists, it would also be nice to have the option to "subscribe" to playlists as well (similar to newpipe).
Once a playlist is subscribed to, every time the app opens, the playlist could check for new videos and fetch 'em.

I already mentioned this in #319 but since that issue got buried and it was too general, I figured I should repost this one particular request.

Thanks in advance.

[grayjay-support-FUTO]

Duplicate of #857.