When a user changes their destination station or checks into a new train
while work is updating data for their in_transit entry, values for no longer
valid database entrie would be entered.

As a side effect, this turns the response of GET /journey/edit (which should
never happen in the first place) from an exception into a 404 error (#36)

The issue manifested itself by the ioloop using 100%cpu without making any
progress.

changed color of symbol in legend

Closes #30