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
Remove magic numbers as per review
Merge pull request #1116 from garvankeeley/1089-more-zoom-for-small-screen

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