model.airport: add valid_until column
model.airport: make by_* aware of the date
lib.xcsoar_.analysis: use flight date to get takeoff/landing airport
scripts/import_welt2000: update database instead of replacing everything
Airports: replace surface 'gras' by 'grass'

See 428ee253594dfb530b197eff6ba0707dcc79e0d5
model.*: add explicit SRID to geometry columns

They somehow got lost?
mapserver: add gray airport icons for closed/removed airports