Use undefined instead of void

We originally wrote `undefined` in CoffeeScript.

CoffeeScript compiled it to `void 0` when generating the JavaScript
files. However, the reason to do so was the `undefined` variable was
mutable before ECMAScript 5. Using `undefined` is more intuitive, and
nowadays there's no reason to use `void 0`.

app/assets/javascripts/watch_form_changes.js

@@ -20,7 +20,7 @@
       }
     },
     initialize: function() {
-      if (App.WatchFormChanges.forms().length === 0 || App.WatchFormChanges.msg() === void 0) {
+      if (App.WatchFormChanges.forms().length === 0 || App.WatchFormChanges.msg() === undefined) {
         return;
       }
       $(document).off("page:before-change").on("page:before-change", App.WatchFormChanges.checkChanges);