fix(frontend linting): var now forbidden

* Prevent the use of var in JavaScript

Fixes #82

frontend/.eslintrc.js

@@ -50,6 +50,7 @@ module.exports = {
         "terms": ["todo", "fixme", "any other term"],
         "location": "anywhere"
       }
-    ]
+    ],
+    "no-var": "error",
   }
 };