Remote control from another device or with an API

[Evanaellio]

When playing a playlist on a desktop computer, or mobile device connected to an audio setup, it would be practical to be able to remote control the player from another device (add/remove people, play/pause, ...)

Possible technical implementation :

• HTTP long polling (frequent API requests to check if something needs to be updated) -> consumes processing and bandwidth even when nothing happens
• Websockets (with Django Channels) -> might disconnect if the user turns off his screen on mobile (Android devices keep reconnecting when screen is turned off primus/primus#350)