[openstreetmap-website] Use a hash anchor for location/zoom persistence (#378)

[John Firebaugh]

Yeah, significant figures was all I could think of too. Seems fragile; let's leave it for now. If we find a compelling use case for making zoom optional and a good way of determining a default, it will still be possible to check if there are two slash-separated components versus three.

The hash now looks like `#map=z/lat/lon&layers=x`. Note (N) and data (D) layers are included, and there's a special case that `layers=M` is dropped -- in other words, if you have the default Mapnik base layer and no overlays, there's no `layers` parameter clutter.

@tomhughes Can we get a test deploy of this please?

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