[openstreetmap-website] Better description and image when posting the link on Facebook or other platforms (#855)

[michaelzangl]

When sharing a link on facebook (or any other social platform that uses previews), you only see the Openstreetmap name and icon.

I explain what I expect with this link:
http://www.openstreetmap.org/?mlat=49.01344&mlon=8.40025#map=18/49.01344/8.40025

I would expect an explanation on what is displayed there (like the where-am-i link) and a nice image indicating the position on the map. That way, you can nicely post Links to the osm website.
![osm1](https://cloud.githubusercontent.com/assets/5981823/5343958/15b0b776-7f0b-11e4-9dd1-c91a345ee4ab.png)
If you see the first link on a forum post, you just think it is advertisement for a website or a hint to use a map an see for yourself.
If you see the second link, you know that the person only wants to share a position with you, and you have the right expectations on what you will see when clicking the link.

The cost would be a Nominatim lookup when getting any page with a mlat/mlon argument, adding a <link rel="image_src"/> tag to the page and generating a 200px x 200px image when Facebook requests it.

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/issues/855
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