1089: For screens less than 500px wide, allow higher zooming on the map. 

On the 320px screen I tested, full zoom out was 20x20km, previously it was 5x5km. 
Screens over 500px wide are unchanged.

#1089
11 and 13 should be pulled out as constants. Maybe as SMALL_SCREEN_MIN_ZOOM and LARGE_SCREEN_MIN_ZOOM?