Add a streaming-only mode for subscriptions

[Tzahi12345]

If a YouTube channel is large enough, users may not want to download all the videos. Instead, they may just want a list of videos, and when they click a particular one, it gets streamed directly to the user.

I need to do more research on the viability of this, but this excerpt from the youtube-dl README makes it seem possible:

If you want to play the video on a machine that is not running youtube-dl, you can relay the video content from the machine that runs youtube-dl. You can use -o - to let youtube-dl stream a video to stdout, or simply allow the player to download the files written by youtube-dl in turn.

A refactoring would need to be done of the subscriptions system. It would also make sense to have "streaming-only mode" be a playlist or channel-specific setting, rather than a global setting.