Include map url(s) in full map data download

[hholzgra]

The inofficial geojson export (see issue #78) contains a shortUrl property pointing back to the maps web URL.

I'd like to have the same in the files exported by "Share -> Download data -> full map data" in the web UI.

Background: I added support for importing such files as overly in my MapOSMatic fork a while ago:

https://blog.osm-baustelle.de/index.php/2018/02/20/umap-overlay-support-added/

and recently also added support for adding an optional QR code to the map output.

For Umap imports I'd like to have that QR code automatically link back to the actual Umap map in the print output by default. For that I'd need to have the actual map URL (or at least the umap_id) included in the downloaded file

[yohanboniface]

Makes sense, I'll add that! :)