Add download format GPX
Please provide a second download format: .GPX
Reason: Unlike GIS-software like QGIS or JOSM, other software or GPS
devices are not able to load Geojson files. An universal file format
which all of this other software and devices are able to read is GPX.
Would be great if we can choose between Geojson and GPX for download so
we don't have to use external file converters.