logout: Move logout form from the ".app" div to the body.

The hidden logout form facilitates logging out from the app by providing
a form with the CSRF token required for the csrf_token protection in
Jinja2.

This commit moves this form from the ".app" div to the body,
where it is more appropriately placed in the DOM.

This is a prep commit for #27550.

templates/zerver/app/index.html

@@ -279,9 +279,10 @@
         <div class="column-right" id="right-sidebar-container">
         </div><!--/right sidebar-->
     </div><!--/row-->
-    <div class="hidden">
-        <form id="logout_form" action="/accounts/logout/" method="POST">{{ csrf_input }}
-        </form>
-    </div>
+</div>
+
+<div class="hidden">
+    <form id="logout_form" action="/accounts/logout/" method="POST">{{ csrf_input }}
+    </form>
 </div>
 {% endblock %}