Added length attribute to tracks

Enhancement as discussed in https://trello.com/c/cou5RNHR/6-app-www-server-geodesic-distance-calculations
Goes together with https://github.com/enviroCar/enviroCar-app/pull/251
I tested the upload successfully with my own enviroCar server and app. Will work only for tracks uploaded with the respective new app version. Other (older) tracks will show length = 0.