Disallow let if value never changes. Use const instead

Author: dkundel

.eslintrc.json

@@ -24,6 +24,7 @@
         "require-jsdoc": 0,
         "no-useless-escape": 0,
         "guard-for-in": 0,
-        "new-cap": 0
+        "new-cap": 0,
+        "prefer-const": "error"
     }
 }