[openstreetmap/openstreetmap-website] fix for #1239 add public gps tiles layer for debugging purposes (#1406)

[Elliott Plack]

Thanks for adding. This looks great. 💯 
![chrome_2017-01-04_14-46-28](https://cloud.githubusercontent.com/assets/1827881/21658206/a5e2d634-d293-11e6-9848-0d6634c24736.png)


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