Merge pull request #1956 from SukkaW/replace-polyfill-io

Replace `polyfill.io`

Author: rjzupkoii

_includes/head/custom.html

@@ -26,7 +26,7 @@
 
 
 <!-- Support for MatJax -->
-<script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
+<script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=es6"></script>
 <script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"></script>
 
 <!-- end custom head snippets -->