Adding a notion of instances per mapAdding a notion of instances to room

The URL signature becomes:

https://workadventu.re/_/[instance]/[path_to_map.json]

This allows us to create many instances of the same map (and therefore to create several different worlds for different people)

An exit on a map can target another "instance" by passing the "exitInstance" property.

back/src/Controller/MapController.ts

@@ -20,7 +20,8 @@ export class MapController {
     getMaps() {
         this.App.get("/maps", (req: Request, res: Response) => {
             return res.status(OK).send({
-                mapUrlStart: req.headers.host + "/map/files" + URL_ROOM_STARTED
+                mapUrlStart: req.headers.host + "/map/files" + URL_ROOM_STARTED,
+                startInstance: "global"
             });
         });
     }