Deep-freeze the remaining configuration objects

Since I did not explain deep-freezing in the last commit..it essentially
makes an entire object read-only, meaning that instead of mangling important
data, badly written code will throw an error!

constants.js

@@ -1225,3 +1225,10 @@ const powerups = {
     }
   }
 }
+
+deepFreeze(prodUpgradeText);
+deepFreeze(prodAllUpgradeText);
+deepFreeze(clickUpgradeText);
+deepFreeze(helperUpgradeText);
+deepFreeze(clickVictimUpgradeText);
+deepFreeze(powerupFreqUpgradeText);