[openstreetmap/openstreetmap-website] Add OPNVKarte (#2670)

[Tom Hughes]

This is what the layer switcher would look like with the names @simonpoole suggested:

![Screenshot_2020-06-28 OpenStreetMap](https://user-images.githubusercontent.com/147741/85954661-0b490800-b971-11ea-8c14-7efb4f21cc11.png)

So we will definitely need to change the font size or something if we go for such long names.

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