forms.flight: add flight.id to every flight associated form
views.upload: replace None value with 0 in UploadUpdateForm

The pilot_id and co_pilot_id fields are int only.
forms.validators: add CheckPilot validator

Check whether pilot or co-pilot are airborne already within
the flight's takeoff and landing.
views.upload: validate pilot_id and co_pilot_id

Make sure both pilot and co-pilot are airborne in one flight only.