Allow user to subscribe to playlist (#17)

[DmitrySandalov]

No description provided.

[omarroth]

Fantastic! Thank you!

Unfortunately I don't have the time to give this the review it deserves. I've invited you as a collaborator, please feel free to merge when you think it's ready.

[DmitrySandalov]

This has been deployed to my own instance for a few weeks and it looks to be working as expected.

The current implementation adds control buttons for playlists created by others to subscribe/unsubscribe. After subscribing to playlist it is displayed on the Playlists page under separate header 'Saved Playlists' (as on YT). Own playlists header is renamed to 'Created Playlists'.

There is space for further improvement, but I'd rather leave this commit as is and work on other related issues separately:

• thumbnails for saved playlists
• hiding already watched videos in saved playlists
• updates when the remote playlist changes (video_count, description, etc)
• showing new videos from saved playlists on the main subscription page

So, if you don't mind, I'll go ahead and merge this request. Thank you.

[github-actions]

This pull request has been automatically locked since there has not been any activity in it in the last 30 days. If you want to tell us about needed or wanted changes or if problems related to this code are discovered, feel free to open an issue or a new pull request.