[BUG] Handle concurrent splash screen connections

[vicwomg]

Describe the bug

If multiple splash screens are open, they go out of sync with one another. This results in them messing up the playback state, and often causing premature skips

To Reproduce
Steps to reproduce the behavior:

1. Open two tabs with splash screens
2. Play a file
3. Skip a track
4. Track will skip on one splash screen, but not the other

Same goes with volume changes, pause/play.

Expected behavior

Both splash screens should be mostly in sync, within a reasonable time margin of a couple of seconds

Additional context

I propose we primarily use the now_playing object for synchronization. This will also eliminate the need for much of the now_playing_command handling, as the clients just react to the server state instead of the command coming from the server.

{
    "now_playing": null,
    "now_playing_user": null,
    "now_playing_command": null,
    "up_next": null,
    "next_user": null,
    "now_playing_url": null,
    "is_paused": true,
    "transpose_value": 0,
    "volume": 0.85,
    "hash":  null
}

We can keep a local version of this object on the splash screen js. If they are not equal (I believe we can use the hash value to determine this), then execute the necessary action to synchronize them.

Examples:

• If video is playing and nowplaying.is_paused == false, then pause the track
• If volume does not match, then set the client volume to match
• If video is playing and now_playing is null, then stop the video

I believe that only skip and restart would be necessary to send over now_playing_command. Of these, I think restart might be the trickiest to implement.