Support loading a trip without specifying a user
Support loading a trip without specifying a user

If no user is given but a trip is set, it would previously overwrite that
information as it would require the user. But if the trip is the ID it is
unique over all trips of every user, so the user is not necessary and it can
imply the user ID using the trip ID.

espinosaluis/TrackMeViewer,espinosaluis/TrackMeViewer