upgrade to bootstrap 4, complete rewrite of template (#629)

_includes/gtag.html

@@ -1,11 +1,10 @@
 {% if site.gtag %}
 <!-- Global site tag (gtag.js) - Google Analytics -->
 <script async src="https://www.googletagmanager.com/gtag/js?id={{ site.gtag }}"></script>
-    <script>
-          window.dataLayer = window.dataLayer || [];
+<script>
+  window.dataLayer = window.dataLayer || [];
   function gtag(){dataLayer.push(arguments);}
   gtag('js', new Date());
-
   gtag('config', '{{ site.gtag }}');
 </script>
 {% endif %}