Improvement / Limit burst songs adding

[Bratanov]

Set a hard limit of how many songs can be added per minute/few seconds, so the playlist can't be flooded with Djordjano's all time playlist songs.

How to reproduce:

1. Go into S Мюзик's YouTube channel: https://www.youtube.com/channel/UCEvaHPE8HZbIyAbQD_maekQ/videos
2. Click Load more below until all videos are loaded.
3. Execute the following code in the console:

   {
   let songs = []
   for(let song of $$(".spf-link")) {
   if(song.href && song.href.indexOf("watch?") !== -1) { 
     songs.push(song.href) 
   }
   }
   console.log("[\"" + songs.join("\",\"") + "\"]")
   }

4. Copy console output.
5. Go to community-driven-radio's console. Type in let songs =, Paste and execute.
6. Execute the following code in the console:

   for(let song of songs) {
   socket.emit("new_song", getIdFromYoutubeUrl(song))
   }

7. Have a nice day!

Of course, we could incapsulate the methods, socket connection, even provide a captcha for the input, however I believe burst limit will be the best way to handle this. Limit by IP sounds even better if we can differentiate between remote/local IPs in Node.