lib.xcsoar_.analysis: add phases to temporary flight

Don't add them to the database, otherwise rejecting the flight
after the analysis in views.upload.index_post() would fail.
views.upload: reject flights with future dates
views.upload: store flight.id in UploadUpdateForm
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.
Merge pull request #358 from TobiasLohner/upload

Add some checks to the upload page