Fix type of default map visibility setting.

The UI sets these to the proper enum types; only the default is wrong.
Fix the default and clean up the associated code.

Note that this does minorly break save compatibility and alters default
behavior, since previously we were ignoring the default option. Ignoring
the default looks unintentional since there is no explicit "don't force
this option" setting in the UI.

Existing saves can be fixed simply by changing this option to something
else and then back.

game/settings.py

@@ -1,6 +1,8 @@
 from dataclasses import dataclass, field
 from typing import Dict, Optional
 
+from dcs.forcedoptions import ForcedOptions
+
 
 @dataclass
 class Settings:
@@ -15,7 +17,7 @@ class Settings:
     player_skill: str = "Good"
     enemy_skill: str = "Average"
     enemy_vehicle_skill: str = "Average"
-    map_coalition_visibility: str = "All Units"
+    map_coalition_visibility: ForcedOptions.Views = ForcedOptions.Views.All
     labels: str = "Full"
     only_player_takeoff: bool = True  # Legacy parameter do not use
     night_disabled: bool = False