nodeMaps should probably point to json files instead of map location

As the ff map (http://www.freifunk-karte.de/) and maybe some other tools need the json files to get their data, they need a location to search for. So they use the nodeMaps entry from the api and just look in that folder for the node.json file. But the json files for the map (ffmap, meshviewer, ..) dont necessarily need to be in the same location as the map.

So the nodeMaps url field should either contain the url of the json files or there should be another field pointing to the node.json url.

Start with the API response's nodeMaps entry and the node.json files consumed by ffmap, meshviewer, and other tools. Check whether nodeMaps should contain the JSON URL directly or gain a separate node.json URL field, then verify that consumers can locate the files without assuming they share the map location.

Written by the indexing model from the issue text.