Fix syntax highlight initialization

Previously hljs was initialized via a function that relies on the
DOMContentLoaded event, registerd after jQuery's 'ready' event. I assume
that with the recent jQuery update, DOMContentLoaded may not be
guaranteed to fire after 'ready'. Fixed this via vanilla JS initalization.

Fixes: go-gitea#7559

public/js/index.js

@@ -2007,7 +2007,10 @@ $(document).ready(function () {
 
     // Highlight JS
     if (typeof hljs != 'undefined') {
-        hljs.initHighlightingOnLoad();
+        const nodes = [].slice.call(document.querySelectorAll('pre code') || [])
+        for (let i = 0; i < nodes.length; i++) {
+            hljs.highlightBlock(nodes[i]);
+        }
     }
 
     // Dropzone