add GTM script on the head

Author: anmol-algo

src/partials/head-first.hbs

@@ -3,3 +3,12 @@
 <script>(function(l,p){if(l.protocol!==p)l.protocol=p})(location,'https:')</script>
 {{/if}}
 <meta name="viewport" content="width=device-width,initial-scale=1.0">
+<!-- Google Tag Manager -->
+<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push(
+
+{'gtm.start': new Date().getTime(),event:'gtm.js'}
+);var f=d.getElementsByTagName(s)[0],
+j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
+'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
+})(window,document,'script','dataLayer','GTM-MVPNN2');</script>
+<!-- End Google Tag Manager -->