Improve startup.css cloaking

We have seen flickering of content on the Issue boards pages. By
improving our selectors, we can prevent that flickering.

Looking at app/views/layouts/_page.html.haml, the selectors:

- `.content-wrapper > .alert-wrapper` for breadcrumbs, flashes and
everything
- `#content-body` for the content of a page

seem more reliable than `.container-limited` which isn't set on _all_
pages.

app/assets/stylesheets/application.scss

@@ -53,7 +53,5 @@
 }
 
 /* Rules for overriding cloaking in startup-general.scss */
-.container-limited,
-.modal-dialog {
-  display: block;
-}
+@import 'startup/cloaking';
+@include cloak-startup-scss(block);

app/assets/stylesheets/startup/_cloaking.scss

+/**
+  Prevent flashing of content when using startup.css
+ */
+@mixin cloak-startup-scss($display) {
+  // Breadcrumbs and alerts on the top of the page
+  .content-wrapper > .alert-wrapper,
+  // Content on pages
+  #content-body,
+  // Prevent flashing of haml generated modal contents
+  .modal-dialog {
+    display: $display;
+  }
+}