Use the SRI hashes for the live site's assets. (#25282)

_includes/header.html

@@ -14,7 +14,7 @@
 
 <!-- Bootstrap core CSS -->
 {% if site.github %}
-<link href="{{ site.baseurl }}/dist/css/bootstrap.min.css" rel="stylesheet">
+<link href="{{ site.baseurl }}/dist/css/bootstrap.min.css" rel="stylesheet" integrity="{{ site.cdn.css_hash }}" crossorigin="anonymous">
 {% else %}
 <link href="{{ site.baseurl }}/dist/css/bootstrap.css" rel="stylesheet">
 {% endif %}

_includes/scripts.html

 <script src="{{ site.cdn.jquery }}" integrity="{{ site.cdn.jquery_hash }}" crossorigin="anonymous"></script>
 <script>window.jQuery || document.write('<script src="{{ site.baseurl }}/assets/js/vendor/jquery-slim.min.js"><\/script>')</script>
 
-<script src="{{ site.baseurl }}/assets/js/vendor/popper.min.js"></script>
+<script src="{{ site.baseurl }}/assets/js/vendor/popper.min.js"{% if site.github %} integrity="{{ site.cdn.popper_hash }}" crossorigin="anonymous"{% endif %}></script>
 
 {%- if site.github -%}
-  <script src="{{ site.baseurl }}/dist/js/bootstrap.min.js"></script>
+  <script src="{{ site.baseurl }}/dist/js/bootstrap.min.js" integrity="{{ site.cdn.js_hash }}" crossorigin="anonymous"></script>
 {%- else -%}
   <script src="{{ site.baseurl }}/dist/js/bootstrap.js"></script>
 {%- endif -%}