Added a new setting: "Maximum frontline length (km)"

It replaces the FRONTLINE_LENGTH constant, which was used previously. The default setting for the frontline length has been set to match the FRONTLINE_LENGTH value (80 km), but I feel the default should be a bit shorter. Discussion on the topic would be welcome.

game/settings/settings.py

@@ -394,6 +394,15 @@ class Settings:
         min=30,
         max=150,
     )
+    # Mission specific
+    max_frontline_length: int = bounded_int_option(
+        "Maximum frontline length (km)",
+        page=MISSION_GENERATOR_PAGE,
+        section=GAMEPLAY_SECTION,
+        default=80,
+        min=1,
+        max=100,
+    )
 
     # Performance
     perf_smoke_gen: bool = boolean_option(