Resolve "Specific Async Script Loading by using a Page Variable"

What does this MR do?

It makes it possible to set script loading on a specific page to defer. Simply set @page_defer_javascript to true and all scripts will get the defer="defer" attribute.

Why was this MR needed?

To boost performance already on specific pages. Later on we can reuse the javascript_tag overwrite exactly in the opposite way so all scripts are defer except specific ones.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #34904 (closed)

app/views/layouts/_bootlint.haml

+-# haml-lint:disable InlineJavaScript
 :javascript
-  jQuery(document).ready(function() {
-    javascript:(function(){var s=document.createElement("script");s.onload=function(){bootlint.showLintReportForCurrentDocument([], {hasProblems: false, problemFree: false});};s.src="https://maxcdn.bootstrapcdn.com/bootlint/latest/bootlint.min.js";document.body.appendChild(s)})();
-  });
+  window.onload = function() {
+    var s=document.createElement("script");s.onload=function(){bootlint.showLintReportForCurrentDocument([], {hasProblems: false, problemFree: false});};s.src="https://maxcdn.bootstrapcdn.com/bootlint/latest/bootlint.min.js";document.body.appendChild(s);
+  }