mapbg.png should not be in app/style

Currently the map component's less file has a rule for styling the map background:

```
.map {
   background-color: #f9f9f9;
   background-image: data-uri('mapbg.png');
}
```

But the mapbg.png image is in the application-specific style directory (app/style). It really should be in the map component's style directory (app/src/map/style).

But then, the question is: what path should be used to refer to the png file in the data-uri function?

@fredj, any idea?