[openstreetmap-website] Avoid blank map key sidebar (#396)

[Saman Bemel-Benrud]

If there's no available map key on a layer, disable the map key button + include a tooltip that says "map key unavailable".

Right now, user sees an empty sidebar, is confused and/or disappointed.

Here's a quick mock-up:
![screen shot 2013-07-30 at 1 06 10 pm](https://f.cloud.github.com/assets/108094/880379/5cf915ea-f93a-11e2-93e1-0f1e663beba9.png)


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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/issues/396
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