Add date-based loadout restriction.

Follow up work:

* Data entry. I plan to do the air-to-air missiles in the near term. I
  covered some variants of the AIM-120, AIM-7, and AIM-9 here, but there
  are variants of those weapons for each mounting rack that need to be
  done still, as well as all the non-US weapons.
* Arbitrary start dates.

https://github.com/Khopa/dcs_liberation/issues/490

game/settings.py

@@ -28,6 +28,7 @@ class Settings:
     automate_runway_repair: bool = False
     automate_front_line_reinforcements: bool = False
     automate_aircraft_reinforcements: bool = False
+    restrict_weapons_by_date: bool = False
 
     # Performance oriented
     perf_red_alert_state: bool = True