Implement warn about opening external links option

osu.Game/Configuration/OsuConfigManager.cs

@@ -42,6 +42,8 @@ namespace osu.Game.Configuration
                 if (!val) Set(OsuSetting.SavePassword, false);
             };
 
+            Set(OsuSetting.WarnAboutOpeningExternalLink, true);
+
             // Audio
             Set(OsuSetting.VolumeInactive, 0.25, 0, 1, 0.01);
 
@@ -148,6 +150,7 @@ namespace osu.Game.Configuration
         BeatmapSkins,
         BeatmapHitsounds,
         IncreaseFirstObjectVisibility,
-        ScoreDisplayMode
+        ScoreDisplayMode,
+        WarnAboutOpeningExternalLink
     }
 }