Add Google search console verification code

This proves to Google search console that we own this domain, and will let us start getting some more insights about how and when Notify appears in search results.

app/templates/admin_template.html

@@ -17,6 +17,7 @@
   <!--[if IE 8]>
   <link rel="stylesheet" media="screen" href="{{ asset_url('stylesheets/main-ie8.css') }}" />
   <![endif]-->
+  <meta name="google-site-verification" content="niWnSqImOWz6mVQTYqNb5tFK8HaKSB4b3ED4Z9gtUQ0" />
 {% endblock %}
 
 {% block page_title %}