[feature] Allow maps to track mobile co-ordinates

[pandafy]

Depends on #561

OpenWISP allows devices to dynamically update their locations. The map should be able to handle such devices with mobile co-ordinates and update their location promptly on the map.

We shall make use of websockets which streams data of updated locations. We can send data from this websocket when post_save signal is received for Location object.

We will probably need to add a new websocket endpoint for this. Let's also use the ReconnectingWebSocket library like in other OpenWISP Modules (notifications and controller).