Allow relative URLs in layer configuration

[lonvia]

It would be nice if the UI allows relative URL names in the global layers.json and each layer.json file. These URLs should then be interpreted as relative to the position of each configuration file.

With this change there would no longer be a need for the Nominatim-Data-Analyser to know about where the data is served when generating its layers.json file. Makes development deployments a tiny bit easier.