Add feature flag for new ground unit recruitment.

This is going to render most campaigns unusable because they won't have
any places to spawn ground units, so flagging off for now.

https://github.com/Khopa/dcs_liberation/issues/986

game/settings.py

@@ -33,6 +33,10 @@ class Settings:
     disable_legacy_aewc: bool = False
     generate_dark_kneeboard: bool = False
 
+    #: Feature flag for new ground unit behavior. Old campaigns are sufficiently broken
+    #: that we need to implement this conditionally for the time being.
+    enable_new_ground_unit_recruitment: bool = False
+
     # Performance oriented
     perf_red_alert_state: bool = True
     perf_smoke_gen: bool = True