Preserve script load/execution order (#48465) (#48591)

We need to either disable async loading, or defer exection. In IE11,
we're running into an issue where the logout page is executing before
the vendors file.

Signed-off-by: Tyler Smalley <tyler.smalley@elastic.co>

src/legacy/ui/ui_render/bootstrap/template.js.hbs

@@ -46,7 +46,7 @@ if (window.__kbnStrictCsp__ && window.__kbnCspNotEnforced__) {
     function createJavascriptElement(path) {
       var dom = document.createElement('script');
 
-      dom.setAttribute('async', '');
+      dom.setAttribute('defer', 'defer');
       dom.addEventListener('error', failure);
       dom.setAttribute('src', file);
       dom.addEventListener('load', next);