[openstreetmap-website] Facebook share image (#739)

[Paul Norman]

> We can implement a new function to generate thumbnail on-fly, containing small version of current view of the map and then place it in html's og:image.

This probably belongs outside the rails port, which doesn't do anything with tile rendering. 

The tile.openstreetmap.org servers provide the /cgi-bin/export endpoint, but it's [not suitable for this](http://wiki.openstreetmap.org/wiki/Tile_usage_policy).

MapQuest Open provides the full featured [Static Map](http://open.mapquestapi.com/staticmap/) endpoint which would work for this.

@gravitystorm, do your layers have any suitable similar endpoint?

For layers without a static endpoint, the best option might be to take the tile from four zoom levels up, to get a 256x256 preview.


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