Add cdnjs MathJax2 URL function

vle_specific.php

@@ -193,6 +193,10 @@ function stack_get_mathjax_url(): string {
     return 'https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML';
 }
 
+function stack_get_mathjax2_url_cdnjs() {
+    return 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.7/MathJax.js?config=TeX-MML-AM_CHTML';
+}
+
 /*
  * Gets the url for MathJax 3.
  */