Enable block-scoped-var lint rule

This will ease migration to let/const in future.

.eslintrc

@@ -11,6 +11,7 @@
     "Promise": false,
   },
   "rules": {
+    "block-scoped-var": "error",
     "no-console": [
       "error",
       { allow: ["warn", "error"] },

h/static/scripts/app.js

 'use strict';
 
-require('./polyfills');
 var queryString = require('query-string');
 
+require('./polyfills');
+
+var raven;
+
 // Initialize Raven. This is required at the top of this file
 // so that it happens early in the app's startup flow
 var settings = require('./settings')(document);
 Object.assign(settings, queryString.parse(window.location.search));
 if (settings.raven) {
-  var raven = require('./raven');
+  raven = require('./raven');
   raven.init(settings.raven);
 }