Start location of towing aircraft is not set correct

[pschuler78]

When changing start location of glider flight, the start location is not set to the same start location on towing flight when already a start location is set.

Steps to reproduce:

1. Create new glider flight with all form data
2. Set start location of glider flight (if not already set). This will set the landing location and all locations of towing flight to the same, except when already a location was set.
3. When changing the start location of the glider flight, the start location is not changed to the same start location.

So the problem looks like that the towing start location is not changed when already a start location is set.