Move unit delivery ownership out of the UI.

Breaks save compat, but we need to have this knowledge outside the UI so
we can know whether or not we can ferry aircraft to the airfield without
overflowing parking.

game/event/event.py

@@ -11,12 +11,12 @@ from dcs.unittype import UnitType
 from game import db, persistency
 from game.debriefing import Debriefing
 from game.infos.information import Information
-from game.operation.operation import Operation
 from game.theater import ControlPoint
 from gen.ground_forces.combat_stance import CombatStance
 
 if TYPE_CHECKING:
     from ..game import Game
+    from game.operation.operation import Operation
 
 DIFFICULTY_LOG_BASE = 1.1
 EVENT_DEPARTURE_MAX_DISTANCE = 340000