Refactored doc-switcher.js

- Simplified code
- Stopped using jQuery
- Moved refactored code to `app.js`

This is the first in a series of patches that will move JavaScript code
out of `require.js` modules and into a single file while also
refactoring.

This patch should bring no user-facing changes.

djangoproject/static/js/app.js

@@ -0,0 +1,5 @@
+document.querySelectorAll('.doc-switcher li.current').forEach(function (el) {
+  el.addEventListener('click', function () {
+    this.parentElement.classList.toggle('open');
+  });
+});

djangoproject/static/js/main.js

@@ -23,10 +23,6 @@ define(function() {
         mods.push('mod/version-switcher');
     }
 
-    if (hasClass('doc-switcher')) {
-        mods.push('mod/doc-switcher');
-    }
-
     if (hasClass('doc-floating-warning')) {
         mods.push('mod/floating-warning');
     }

djangoproject/templates/base.html

@@ -165,6 +165,7 @@
       };
     </script>
     <script data-main="{% static "js/main.js" %}" src="{% static "js/lib/require.js" %}"></script>
+    <script src="{% static "js/app.js" %}"></script>
     {% block body_extra %}{% endblock body_extra %}
   </body>
 </html>