Allow manual distance override

[TiStrutzi]

I would like to be able to manually change the travel distances in instances where the automatic calculation results in incorrect data.

E.g. I created a few manual check-ins to/from Manchester Airport recently, which for some reason was placed in the middle of the ocean, resulting in a 100km bus journey being shown as 7400km. I would like to keep the check ins, but doing so will completely mess up my statistics.

Another usage could be taking certain journeys out of the statistics. E.g. I would like to check into flights for the "telling my friends what I'm up to" aspect of Träwelling, but not for the "I'm a train nerd and want to know how much I use them" aspect. Setting the flights distance to 0km would take them out of the calculation but still allow the journey to be seen.