Implement selection chests for BP rewards.

src/main/java/emu/grasscutter/data/excels/BattlePassRewardData.java

@@ -17,11 +17,12 @@ public class BattlePassRewardData extends GameResource {
 
     @Override
     public int getId() {
-        return this.level;
+        // Reward ID is a combination of index and level.
+        // We do this to get a unique ID.
+        return this.indexId * 100 + this.level;
     }
 
     @Override
     public void onLoad() {
-    	
     }
 }