Add gpx export for routes

[yvecai]

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