Add London Airport and Million Mile Challenges

app/Models/Airport.php

@@ -33,6 +33,10 @@ class Airport extends Model
         'display_name',
     ];
 
+    const array LONDON_AIRPORTS = [
+        'LHR', 'LGW', 'STN', 'LTN', 'LCY', 'SEN',
+    ];
+
     protected function displayName() : Attribute{
         return Attribute::make(
             get: function () {