Add a feature flag for pilot limits.

This doesn't currently interact very well with the auto purchase since
the procurer might by aircraft that don't have pilots available. That
should be fixed, but for the short term we should just default to not
enabling this new feature.

gen/flights/ai_flight_planner.py

@@ -178,7 +178,7 @@ class AircraftAllocator:
                     aircraft, task
                 )
                 for squadron in squadrons:
-                    if squadron.number_of_available_pilots >= flight.num_aircraft:
+                    if squadron.can_provide_pilots(flight.num_aircraft):
                         inventory.remove_aircraft(aircraft, flight.num_aircraft)
                         return airfield, squadron
         return None