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

[polarbearing]

I don't get the tiles delivered: 
https://b.gps-tile.openstreetmap.org/lines/16/35185/21489.png
```
b.gps-tile.openstreetmap.org uses an invalid security certificate. 
The certificate is only valid for *.tile.openstreetmap.org
```

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