Add cloak-startup class and apply to mobile overlay

We currently have several specific elements cloaked to prevent
flashing when using startup.css. This MR introduces a generic
class which can be applied to any element to achieve the same.

Here we apply it to the mobile top nav overlay to fix broken
rendering in certain race/network conditions.

Changelog: fixed

app/assets/stylesheets/startup/_cloaking.scss

@@ -2,6 +2,8 @@
   Prevent flashing of content when using startup.css
  */
 @mixin cloak-startup-scss($display) {
+  // General selector for cloaking until ready
+  .cloak-startup,
   // Breadcrumbs and alerts on the top of the page
   .content-wrapper > .alert-wrapper,
   // Content on pages

app/views/layouts/nav/_top_nav_responsive.html.haml

@@ -2,4 +2,5 @@
 - view_model = top_nav_responsive_view_model(project: @project, group: @group)
 
 .top-nav-responsive{ class: top_class }
-  #js-top-nav-responsive{ data: { view_model: view_model.to_json } }
+  .cloak-startup
+    #js-top-nav-responsive{ data: { view_model: view_model.to_json } }