Add gpx export for routes #33
Description
In case somebody wants to have a look at this, the geometry in WKT is available here:
https://github.com/yvecai/opensnowmap.org/blob/40abd5b305875f241f5032a959ee711d7b0416bb/www.opensnowmap.org/js/opensnowmap_mobile.js#L2546
The downloading button should probably be created here:
https://github.com/yvecai/opensnowmap.org/blob/40abd5b305875f241f5032a959ee711d7b0416bb/www.opensnowmap.org/js/opensnowmap_mobile.js#L2555