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`.

.eslintrc.yml

@@ -36,6 +36,7 @@ rules:
   no-spaced-func: error
   no-tabs: error
   no-trailing-spaces: error
+  no-void: error
   no-whitespace-before-property: error
   object-curly-spacing:
     - error

app/assets/javascripts/cookies.js

@@ -1,10 +1,9 @@
-// Generated by CoffeeScript 1.12.6
 (function() {
   "use strict";
   App.Cookies = {
     saveCookie: function(name, value, days) {
       var date, expires;
-      expires = void 0;
+      expires = undefined;
       if (days) {
         date = new Date;
         date.setTime(date.getTime() + days * 24 * 60 * 60 * 1000);

app/assets/javascripts/forms.js

@@ -18,7 +18,7 @@
       $(".js-toggle-link").unbind("click").on("click", function() {
         var toggle_txt;
         $($(this).data("toggle-selector")).toggle("down");
-        if ($(this).data("toggle-text") !== void 0) {
+        if ($(this).data("toggle-text") !== undefined) {
           toggle_txt = $(this).text();
           $(this).text($(this).data("toggle-text"));
           $(this).data("toggle-text", toggle_txt);

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);